Preheat the oven to 160 °C (320 °F).

Grease the bund pan form (10 x 3 1/2 inches / 25x9 cm) with buttter and sprinkle with flour.

Sift the flour with baking powder. Mix cocoa powder with rum. Divide egg yolks from egg whites.

With a stand mixer, mix butter with powdered sugar, vanilla extract, lemon zest and salt for 5 minutes until creamy (if using hand mixer mix 10 minutes). Add one egg yolk at the time and mix for 1 more minute.

Lightly beat egg whites until frothy (around 30 seconds), add granulated sugar and mix until soft peaks form (do not overmix). Carefully fold in sifted flour with baking powder, butter mixture, carefully mix with spatula until smooth.

Divide the mixture into 2 equal parts and in one part add cocoa powder with rum.

Alternately spoon white and cocoa mixture into prepared bundt pan form, until you use up both mixtures. Use fork or knife to swirl the cocoa batter into the white batter to incorporate it in a marble effect.

Bake in preheated oven (lower rack) for 50 minutes to 1 hour, depending on your oven or until an inserted wooden pick comes out clean.

Remove the cake from the oven, allow to cool for 10 minutes. Remove the cake from the mold and cool it to room temperature. Sprinkle with powdered sugar.