Mix butter until peaks form. Add sugar and salt. Gradually add eggs and stir until mixture is light. Mix flour with baking powder, stir in with milk. Divide dough between two bowls.
Preheat oven to 180°C top/bottom heat (convection 160°C).
Vanilla dough: Stir vanilla into one half of the dough.
Chocolate dough: Mix cocoa powder and milk into the other half of the dough.
Put vanilla dough into the paper-lined form, add chocolate dough on top, pull through spirally with a fork.
Bake on the lowest rack of the preheated oven for 45-55 minutes. Remove, let stand briefly in the form, remove from the form, let cool on a cake rack.