Preheat oven at 350°F with the rack in the middle. Line an 8-inch round 3-inch high cake pan (or 2 loaf pans) with parchment, butter and flour.

Sift flour, baking powder and salt in a bowl. Set aside.

Cream butter and 155 gr sugar in the bowl of a stand mixer fitted with the paddle attachment till light (very pale) and fluffy. Gradually add egg yolks into the butter and beat on medium speed till just incorporated. Add vanilla extract. Beat until well combined.

Gradually add the sifted dry ingredients into the butter mixture and beat on medium speed till just incorporated.

In a separate bowl, whisk the egg whites till foamy. Add 100 gr sugar in 3 batches. Beat until stiff peaks form.

With a spatula, fold in the meringue into the egg yolk mixture in 3-4 batches.

Mix the cocoa powder and warm water until well combined. Add 1/4 of batter into it. Mix well.

Pour batter into the cake pan, alternating between the two mixtures, starting with the yellow batter and ending with the cocoa mixture. Tap 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.