Custard Sheet Cake, the All-Time Favorite Traybake
·Preheat the oven
Preheat the oven to 170 °C.
Make the custard
Heat the milk (750ml) in a saucepan and sweeten with sugar. Once the milk is hot, almost boiling, stir in the remaining milk in which you've whisked both vanilla custard powders. Stir until it thickens, then keep cooking over a gentler heat for about five more minutes, until the custard is nicely thick.
Whisk the batter
Whisk the eggs and sugar together until foamy. Add the oil, water and flour mixed with baking powder. Mix into a smooth, lump-free batter.
Spread the batter on the tray
Pour the batter onto a prepared baking tray (ideally around 26 x 35 cm), either lined with baking paper or greased with oil and dusted with fine plain flour.
Top with the custard
Pour the custard over the batter, spreading it reasonably evenly. It doesn't need to be perfectly even - that's the charm, as bits of batter will peek through the custard once baked.
Bake
Bake in the preheated oven for 40 minutes.
Brush with sour cream
After this time, remove from the oven and immediately brush the sour cream over the whole surface of the cake. Sprinkle with cinnamon sugar and leave to cool completely.
Serve
Cut into squares and serve.
