Melt the butter and sugar in a small pot on the stove. To them add the milk while stirring nonstop until the mixture comes to a boil.
Set aside 1/2 cup of it. Separately, beat 4 egg yolks with the flour and baking powder. To them add the beaten to snow egg whites.
Bake the mixture in an oiled rectangular tray that you've oiled and sprinkled with flour at 356°F (180 °C) for 45 min. Once it cools, smear it with the set aside mixture.