Note: all ingredients must be at room temperaturenot from the fridge or you won't get a fluffy, soft sponge cake. Also feel free to add nuts too. I actually love nuts but run out of them and noticed on the last moment but I normally add tons of nuts to the sponge cake.
In a saucepan boil milk with sugar and a pinch of salt.
Drizzle 500 g flour in a bowl, then pour hot milk and mix well with a wooden spoon until all the flour is incorporated, then leave to cool.
Prepare a yeast bubble: mix the dry yeast with a teaspoon of sugar and one of flour, dissolve with water, then add 2 egg whites and mix well, then let it expand.
In another bowl, put 2 egg yolks 2 whole eggs, vanilla, rum, lemon peel and mix everything well.
When the #1 is cooled (not completely, must be warm), add 500 g flour and knead well with your hands (or using a food processor, I prefer using hands, it always has better results for me) until all the flour is well incorporated.
Add #3 to the above mixture and knead well until all is incorporated.
Stir the egg mixture at #4 and knead well.
Finally put oil little by little and knead well until the dough forms bubbles. Put the dough into a large bowl, cover with a clean towel and let it expand in a warm place until the dough doubles its volume.
Prepare a cocoa Filling: 2 tablespoons cocoa, 100 g sugar, 1 egg white.
After the dough has expanded enough, split it in two and knead each piece of dough by hand on a board greased with oil. Then spread the dough and fill with raisins and cocoa filling. Roll the dough to cover all the filling.
The 2 cakes are placed in sponge/pound cake molds, lined with baking paper. Leave in the mold to expand some more then brush on egg yolk.
The sponge cake is baked at a temperature of 180 C = 350 F for about 30-40 minutes. Poke with toothpicks once in a while to check if it's done or need more baking but make sure you don't keep the lid open for too long.