Bento Cake: A Small Cake for Two – Sponge, Frosting and Simple Decorating

A bento cake (lunchbox cake) is a mini cake that serves one to three people, depending on its size, and is sold and given in a little box like a snack. It's typically 8–10 cm (3–4 in) across and about 7 cm (2¾ in) tall, covered in smooth pastel frosting and topped with a simple drawing or short message – a name, a joke, "for you".

The word bento is Japanese (a boxed meal), but the trend itself began in South Korean bakeries and spread around the world through Instagram and TikTok during the pandemic, when people weren't celebrating in big groups and a small cake for two was enough. That's why it's now a popular gift for birthdays, anniversaries or just because.

For you, that means two pieces of good news: you need few ingredients and very little decorating. There's nowhere to hide on an 8 cm cake, but there's also no need to overdo it – two elements (a border and a message) are enough.

An 8 cm (3 in) bento cake feeds one to two people, a 10 cm (4 in) one feeds two to three. The Korean standard is closer to 10 cm; 8 cm is a little smaller, but a taller cake looks better proportioned at that size and fits easily into an ordinary box.

Practical measurements for an 8 cm cake:

• Sponge: 3 rounds, each 8 cm in diameter and 1.5–2 cm thick

• Frosting between layers: 2 layers of about 40 g each (roughly 1 cm)

• Coating and decorating: about 170 g of frosting

• Total frosting: approx. 250 g per cake

• Finished height: 6.5–7.5 cm

You build everything in the same metal ring, so it pays to have the right diameter. We use a perforated cake ring, Ø 8 cm: you can cut the sponge with it and build the cake inside it. If you want a 10 cm cake, a ring or cutter of that size from any kitchen set will do.

The best way to make bento cake sponge is to bake one thin sheet on a baking tray and cut the rounds out with a ring. Baking in miniature tins works less well: small tins bake unevenly, the sponge domes, and you lose a lot when levelling.

Here's how:

1. Bake a sheet 1.5–2 cm thick on a baking tray or in a roasting tin of about 20 × 30 cm (8 × 12 in). From a sheet like this you can cut 6 rounds of Ø 8 cm, which makes two bento cakes of three layers each.

2. A soft, springy sponge works well here, so try a light Génoise sponge. A thin sheet bakes for less time than a tall cake – keep an eye on it from the 10-minute mark and take it out as soon as it springs back when lightly pressed.

3. Let the sheet cool completely, ideally wrapped in cling film and chilled for 30 minutes in the fridge. A cold sponge warps and crumbles far less.

4. Press the ring straight down and give it a slight twist – don't turn it like a screwdriver, or the edges will tear.

Want a chocolate version? Bake the sheet using the simple chocolate cake recipe and proceed the same way. Don't throw the offcuts away – mixed with a spoonful of frosting, they make little truffle-style balls to go with coffee.

Tip for keeping it moist: small layers dry out quickly. Lightly brush each round with 1–2 tablespoons of sugar syrup (water and sugar 1:1, boiled and cooled), optionally flavoured with vanilla or lemon zest.

A bento cake needs a frosting that's firm, spreads smooth and holds piped designs – ideally mascarpone whipped cream frosting or Swiss meringue buttercream. Plain unstabilised whipped cream slumps on such a small cake and the lettering spreads.

How they compare:

• Mascarpone frosting – 125 g mascarpone, 100 ml of 33% cream (heavy cream) and 25 g icing sugar (powdered sugar) per cake. It's light and almost white (so pastel colours come out clean) and holds well in the fridge. Downside: it softens within an hour out of the fridge.

• Swiss meringue buttercream – the firmest, and it survives transport and several hours at room temperature. Downside: it's yellowish from the butter, so pink turns out more salmon and blue leans green. We explain how to whiten it in the article snow-white buttercream.

• Stabilised whipped cream – closest to the Korean original, with the lightest flavour. It has to be stabilised with gelatine or a whipped cream stabiliser, otherwise it won't last on the cake.

• Ganache with cream – for a chocolate bento cake. To keep it from being soft, it needs more chocolate than a glaze would and a night in the fridge before you whip it.

If you can't decide, you'll find a comparison of all the common frostings in the article types of cake frosting and where each one works best.

A rule for every frosting: coat and decorate with frosting that's cold, but not rock hard. Warm frosting "sweats" on the cake and the piped lines come out blurred.

Build the bento cake right in the ring lined with an acetate strip, let it set in the fridge for 2 hours, and only then coat it. The ring keeps the layers exactly on top of each other, so the cake won't lean – on an 8 cm cake every millimetre of error shows.

1. Stand the ring on a board (a cardboard round or a small plate) and line the inside with a strip of acetate about 8 cm tall. We explain why and how in the article cake collar (acetate sheet).

2. Place in the first sponge round, brush with syrup, and pipe or spread on 40 g of frosting. Keep the edge 3–4 mm lower than the centre so the frosting doesn't squeeze out under the weight.

3. Repeat with the second layer. Place the third round bottom side up, flat side on top – that gives you a perfectly flat top.

4. Put it in the fridge for 2 hours (1 hour is enough with buttercream).

5. Remove the ring, peel off the acetate and coat the cake with a thin crumb coat of frosting. Chill for 15 minutes.

6. Spread on the final 3–5 mm layer and smooth it with a scraper or a bench knife held straight while you turn the cake, not by moving your hand.

Nobody expects bakery perfection from a bento cake: a slightly rustic coating with a little drawing is exactly what makes the style so cute.

A classic bento cake has three things on it: a pastel coating, a border of stars or pearls, and a short message in the middle. Two or three small piping tips and one gel colour are all it takes. Everything else is a bonus.

Which tip for what:

• Open star (JEM #13) – small stars and "shells" around the top edge and the base of the cake. Small stars don't look clumsy on an 8 cm cake; larger stars (#18 and up) suit 15 cm cakes and cupcakes better.

• Round #12 (JEM) – pearls around the edge, dots, hearts and simple five-dot flowers. It's about 7.5 mm, so the pearls come out bold – for finer pearls, use a smaller round tip. It's already too wide for lettering on a small cake.

• PME Supatube round lettering tip no. 1.5 – a thin line for the message and for outlining drawings. On an 8 cm cake you have about 5 cm of space for the message, so the thinnest tip is worth it.

You'll find tips and other tools together in the cake decorating category. If you don't have a tip, you can pipe the message from a baking-paper cone with the point snipped to 1–2 mm – the line will be a bit less even, but it works.

Colours: for a pastel shade, add just the tip of a toothpick of gel colour to 250 g of frosting, mix and leave to rest for 15–30 minutes. Gel colours (we use ProGel by Rainbow Dust) deepen in the frosting over a few hours, sometimes overnight, so mix a shade lighter than you want. Liquid supermarket colours thin the frosting and won't give you a rich shade. For the message, set aside 2–3 tablespoons of frosting and colour it more strongly, or use black. You'll find more tips on colouring frosting in the article decorating kids' cakes: frosting, colours and themes.

Pipe the message on a bento cake with a no. 1.5 tip and cold frosting, no more than 2 lines of 8–10 characters. Longer text won't fit on 8 cm and starts to look cramped.

• First write the message on baking paper and measure how much space it takes.

• Lightly sketch it with a toothpick on the chilled coating – mistakes can be smoothed away.

• Hold the tip just above the surface and let the line of frosting "fall" into place; don't drag it across the cake.

• Press evenly, and at the end of each letter ease off the pressure before you lift the tip.

For a detailed method in both frosting and chocolate, including how to fix a mistake, see the article writing on a cake: how to pipe it in frosting or chocolate.

A bento cake goes in a paper box with a lid that fits it together with its board – for an 8 cm cake, look for a box with inside dimensions of at least 10 × 10 cm and a height of 9 cm. We don't sell boxes or boards; you can find them at a stationery shop or in online shops selling food packaging.

• Put the cake into the box chilled, ideally after an hour in the fridge following decorating – the frosting is then firm and the message won't smudge.

• Put a dab of frosting under the board so it doesn't slide around the box.

• Mascarpone and whipped cream frostings last about an hour out of the fridge, buttercream several hours. For a longer journey, choose buttercream.

If you're making bento cakes to sell, price them fairly: the ingredients are cheap, but the decorating and message take almost as much time as on a large cake.