Rub the cold cubed butter into the sifted flour with your fingers, or use short pulses in a food processor. It's ready when the mixture looks like coarse wet sand with no large pieces of butter left.

Add the sugar with the zest and vanilla, then the egg yolk. Work it only until the dough holds together. It doesn't need to be smooth.

Transfer the dough to the work surface and, using the heel of your hand or a bench scraper, smear it away from you once or twice, briefly. This works in the last bits of butter. Don't press hard and don't repeat.

Press the dough into a disc 2 cm thick, wrap it in plastic wrap, and refrigerate for at least an hour, ideally overnight. A disc chills through in 40 minutes, whereas a ball takes four hours.

Take the dough out of the fridge and let it soften for 5 to 10 minutes, or 15 minutes if it has been in the fridge overnight. Roll it out between two sheets of baking paper to 3 mm. Don't dust with any flour.

Cut the shapes as close together as possible. Dip the cutter in flour, press straight down, and lift. Don't twist. Make an equal number of bottoms and tops with a window cut-out.

Put the tray of shapes in the freezer for 10 minutes or in the fridge for 15 to 30 minutes. This step decides whether the shapes stay sharp after baking.

Bake in an oven preheated to 175 °C for about 8 to 10 minutes. They're done when the edges are only just starting to turn golden and the center is still pale. The cookies keep cooking after you take them out.

Leave the cookies on the tray for 3 to 5 minutes, then transfer them to a wire rack and let them cool completely. Warm cookies break when you move them.

Warm the jam for 10 to 15 seconds in the microwave or for a minute in a small saucepan, just enough to loosen it. Don't let it boil. If it has seeds, strain it through a sieve while warm.

Mix the powdered sugar with potato starch. Lay the top cookies with the window out separately on a sheet of paper and dust them through a sieve. The starch absorbs moisture, so the sugar stays white.

Turn the bottom cookies flat side up and spread on half a teaspoon of lukewarm jam, about 2 mm from the edge. Cover with a sugar-dusted top cookie and press gently.

Pack them into a tin, layering them with paper, and leave in a cool room to rest for at least three days. Resting isn't waiting; it's the final step of the recipe.