Beat the room-temperature butter until pale, add the powdered sugar, vanillin and salt, and continue beating.

Add the eggs one at a time, beating the mixture well after each one.

Sift in the flour with the baking powder and fold in. Transfer a third of the batter to another bowl, add the cocoa and milk, and mix.

Grease a rectangular loaf tin with butter. Pour in the plain batter first, then the chocolate batter on top, and spread it evenly.

Bake for 50-60 minutes in an oven preheated to 170°C, check for doneness with a skewer. Let the finished cake cool, then slice.