Use a separate bowl to beat egg whites on high until it forms a peak and are stiff. Add a pinch of salt to the egg whites, adding to the fluffiness of the cake. Put aside.

In a large mixing bowl, beat the butter with sugars until well blended and sugar is dissolved. Add egg yolks one at a time, continuing to blend together.

In a dry bowl, mix flour and baking powder together.

Alternate adding the 125 ml milk and flour into the sugar mixture slowly. Continue mixing with a wooden spoon. Add in the rum and mix all together. Batter will be slightly thicker than a pancake batter texture.

Fold in the egg whites.

Divide the batter into two parts: 1:2 ratio.

In the bowl with the less batter, sprinkle in cocoa powder and 2 more tbsp of milk. Mix all together.

Preheat the oven 375F.

Grease baking pan.

Alternate adding the vanilla batter with the chocolate batter, one spoonful at a time in the same center spot. Once both batters are transferred into the pan, give the pan a gently shake to level.

Bake in the oven for 25 minutes. Insert toothpick.

Remove, let cool and sprinkle top with a mix of vanilla and powdered sugar.