Prepare all ingredients:

cremer the softened butter along with two thrid of sugar using a whisk.

Separate the eggs. Add the egg yolks to the butter/sugar mixture and continue to mix with the whisk until the preparation thickens.

Add the sifted flour to avoid lumps. Quickly mix.

Beat the egg whites very firm...

... and stiffen them with the rest of sugar.

Gently add the slightly meringued firm whites...

... to the dough.

Divide the dough into 2 even parts.

Flavor one half with cocoa powder.

Keep the other one plain.

Pour the uncoloured dough in a silicone cake mold like "Sili Cook",...

... cover with chocolate flavoured dough...

... and mix with a fork to form the marbling.

Bake in hot oven 180°C.

Bake 30 minutes for mini marbled cakes and a good 50 minutes for a large marbled cake.

When finished, check that the cake is ready by sticking the blade of a stainless knife in it. The cake is baked when the blade comes out dry.

Unmold on a cooling rack or on the work surface.

Leave to cool before slicing and eating.