My version is slightly different. I added vanilla essence to 2/3 of the portion and the other 1/3 mixed with cocoa powder. For the swirling, the cocoa mixture was put in a piping bag and combined in a circular mode.
70g sugar (first portion)
150g melted butter
100g soft/cake flour
1/4 tbsp baking powder
2 egg whites
1 tbsp cocoa powder
1/2 tsp vanilla
Preheat oven to 175C and line a baking pan with butter.
Whisk yolks with first sugar until light and fluffy.
Add milk and melted butter (cool the butter first)
Sift flour with baking powder and mix into yolk mixture.
Whisk egg whites with second sugar to form a glossy peak.
Fold it gently into flour-yolk mixture.
Take about 1/3 of this batter, add cocoa powder and mix.
Put it in a piping bag and spread into a circular mode.
Add vanilla to the other 2/3 and spoon alternately into the prepared baking pan.
Bake at 175C until cake is cooked.
Let it cool down before slicing.