preheat oven to 180'c grease a 20cm cake tin and line the bottom with a circle of greaseproof paper.
beat egg whites until soft peaks, gradually add 50g sugar and beat until stiff.set aside
With a handheld or stand mixer fitted with a paddle attachment, beat the butter on high speed in a large bowl until smooth and creamy, about 1 minute. Add the granulated and sugars and beat on high speed for 3-4 minutes until creamed. Scrape down the sides and up the bottom of the bowl as needed.
Add the egg yolks one at a time, Beat on medium-high speed until combined. Scrape down the sides. With the mixer running on low speed, add the dry ingredients in 3 additions, alternating with the milk and mixing each addition just until incorporated. Do not overmix.
Divide the mixture between 2 bowls. Stir 2 tbsp cocoa mixture into the mixture in one of the bowls. Take 2 spoons and use them to dollop the chocolate and vanilla cake mixes into the tin alternately.
Take a skewer and swirl it around the mixture in the tin a few times to create a marble effect.
Bake the cake for 22-27 minutes, or until a toothpick inserted into the center of one cake comes out clean.
Note that every oven is different – you need to experiment until you find a temperature that is best for baking butter cakes.