Make the custard: In a saucepan, use a wooden spoon to mix the egg yolks with the sugar. Add the cornstarch, stirring constantly until smooth. Gradually pour in the hot milk, flavored with vanilla or grated lemon or orange zest, while stirring. Place over heat and cook, stirring, until it thickens and simmers for a few minutes. Cover with plastic wrap and let cool.

Preheat the oven to 375°F (190°C). Grease two 8-inch (20 cm) cake pans and line them with nonstick parchment paper.

In a large bowl, beat the sugar and softened butter until light and fluffy. In another bowl, beat the eggs well, then add them to the sugar and butter mixture.

While mixing, add the sifted self-rising flour, baking powder, and milk. Mix until you have a smooth, soft batter.

Divide the batter evenly between the two pans. Smooth the tops with a spatula or the back of a spoon.

Bake for 20–25 minutes, until the cakes are golden and spring back when pressed (hence the name "sponge").

Let the cakes cool completely. Spread the sour cherry preserves on one cake layer, then place the second cake on top. Finish with the custard.