METHOD
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preparation
Set a rack in the middle of the oven and preheat it to 170 °C / 340 °F. Grease a 22 cm (9-inch) marble cake pan with butter.
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whisk the egg yolks
Separate the egg yolks from the egg whites. Add the egg yolks to a large bowl and the egg whites to a small bowl. Add softened butter, rum (optional), powdered sugar, vanilla, and grated lemon zest to the egg yolks and beat with an electric mixer until you get a foamy, creamy batter for about 5 minutes.
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beat the egg whites
Beat the egg whites with a pinch of salt and sugar. Beat until stiff peaks form.
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marble cake mixture
Combine flour, baking powder, and a pinch of salt in a bowl. Add half of the flour and half of the milk to the egg yolk mixture and combine, then add the rest of the flour and milk. Mix until you get a smooth mixture. Gently fold in the egg whites.
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add the cocoa
Transfer two-thirds of the batter to the prepared marble cake pan. Stir cacao and two tablespoons of milk into the rest of the batter. Pour the darker batter on top of the lighter one into the pan. Drag the handle of a spoon or fork through the batter in a wave-like motion to get that marble texture.
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bake and serve
Place the pan into the preheated oven. Bake for 50 minutes at 170 °C / 340 °F or until the exterior is golden brown. Bring to room temperature, then serve. Optionally dust with powdered sugar before serving.