This is one of the classics of cake making: gâteau marbré. A lot of recipes will give you a nice-looking but slightly dry cake, so I have always had mixed feelings towards gâteau marbré. Not so with this recipe. Super easy to make, very tasty and with the perfect texture.
For one big loaf (for a smaller loaf, take 2/3 or 3/4 of the recipe)
Melt 350g unsalted butter in a saucepan. Once melted, mix with 400g sugar, then slowly add 6 egg yolks. Sift in 350g of white flour adding 1 pack of backing powder. Add a pinch of salt. Beat up the 6 remaining egg whites until very stiff, then incorporate delicately into the cake batter (using a spatula).
Once everything is thoroughly combined, divide the batter into two parts. Sift 40-50g dark unsweetened cocoa powder into one of the two parts. This will be your chocolate batter.
Now fill in a loaf pan lined with a non-stick baking sheet. You can either alternate the light and dark batter when filling the pan, or you can first fill in the dark batter, then lay the light one on top of it and mix with a fork in a circular movement, bringing batter from the bottom to the top and vice versa.
Bake for approximatively 60 min in a preheated oven at 200°C. If you find the top of the cake browns too quickly you can continue the baking with a tin foil placed on top. You know the cake is ready when a toothpick inserted into the middle of the cake comes out clean.
Remove the cake from the pan and let it cool completely. It is actually better if you do it a night ahead before serving.
The cake can be kept in a tin foil for up to a week at a cool and dry place.
Recipe adapted from Le Grand Larousse gastronomique