Preheat oven at 160°C (convection fan on) with a wire rack in the middle. Line an 8-inch round cake tin with parchment, butter and flour. 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 the egg yolks with the sugar till light. Gradually add this into the butter and beat on medium speed till just incorporated. In a separate bowl, with a hand mixer, whisk the egg whites till stiff peaks form. With a spatula, fold in the meringue into the egg yolk mixture in 3-4 batches. Add in the dry ingredients gradually until just incorporated.
Divide the batter into two, 3/4 in one batch and the rest in another bowl. Sieve cocoa powder over the lesser batch 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. Rap pan on counter several times to eliminate air bubbles. With a long skewer, swirl around the batter to create the marble effect. Bake for 40-55 minutes or until a tester inserted into the middle of the cake come out clean.
Cool on wire rack completely before serving. Cake keeps at room temperature up to three days and can be frozen up to 3 months.