Recipe adapted from Baking For Happy Kids
4 large eggs, 60g each (weight with shell) separated
90g caster sugar
175g unsalted butter
135g all purpose flour or cake flour
3/4 tsp baking powder
3/4 tsp pure vanilla extract
10g (1 tbsp) good quality cocoa powder
1/4 tsp salt
Preheat oven at 180°C or 160°C (fan forced) with a wire rack in the middle. Line one 14 cm round cake tin with baking paper. Sift flour, baking powder and salt in a bowl. Set aside.
Cream butter in the bowl of a stand mixer fitted with the paddle attachment till light and fluffy. In a separate bowl, whisk egg yolks with sugar until light and combined. Gradually add egg yolk mixture into the butter and beat on medium speed till just incorporated. In a separate bowl, with a hand mixer in medium, whisk egg whites until stiff peaks form, in about 10 mins. Use a spatula to fold in the egg white mixture into the egg yolk mixture in 3-4 batches. Add in vanilla extract and dry ingredients. Stir gradually until just incorporated.
Divide the batter into two, 3/4 in one portion and the rest in another portion. Sieve cocoa powder over the smaller portion and fold to mix well. Pour batter into tin, alternating between the two mixtures, starting with the yellow batter and ending with the cocoa mixture. Tap the filled tin on kitchen top gently to eliminate any air bubbles. With a long skewer, swirl around the batter to create the marble effect. Bake for 40-55 mins or until a tester inserted into the middle of the cake come out clean.
Allow cake to cool slightly in the tin for about 10 mins and transfer onto a wire rack to cool completely before serving.