Cream softened butter with sugar and salt in three additions. Add egg yolks one at a time, then gradually mix in egg whites in three additions.

Sift cake flour and baking powder, then fold gently using a cutting motion. Add milk and mix until combined.

Separate 1/5 of the batter into another bowl. Mix in sifted cocoa powder and water until smooth.

Layer plain batter and cocoa batter alternately in a buttered mold. Swirl with a toothpick or chopstick for a marble effect. Shape into a dome and score the center with a buttered spatula.

Bake in a preheated oven at 170°C for 40 minutes. For mini pound cakes or muffin pans, bake for 25-30 minutes. Test with a skewer for doneness.