Marble cake, think of how easy this cake is to make

Preparation method
Preheat the oven to 180°C.
Mix the creamy butter with the sugar until it forms a homogeneous cream.
Add the yolks (reserve the whites) and beat well.
Add the milk and continue beating until well incorporated.
Gradually add the flour and yeast, mixing until you get a smooth dough.
In another bowl, beat the egg whites until they form firm peaks.
Gently fold the egg whites into the batter using a spatula.
Divide the dough into two parts: in one of them, add the cocoa powder and mix well.
In a greased and floured pan, pour a layer of white dough, followed by the cocoa dough and finish with the rest of the white dough.
Bake for about 50 minutes or until a toothpick inserted comes out clean.