In a food processor, mix butter, sugar, and yolks

add baking powder, milk, flour, and vanilla sugar.

oil a 9x11 baking pan, line with wax paper and pour half of the batter, bake till light brown.

Meanwhile, add cocoa to the other half of the batter and mix well.

When first half is baked, remove from the oven, and immediately, spread jam over it.

Pour the second half of the batter on top and bake.