Butter and line a 2.5l bundt tin. Heat the oven to 180°C/160°C fan/gas 4. Beat the egg yolks with the sugar until lighter in colour, then beat in the soft butter followed by the milk or cream.

In another bowl, beat the egg whites until they form soft peaks, then add the vanilla sugar and beat until stiff. Beat the flour and baking powder into the yolk mixture, then fold in the egg whites.

Pour half of the batter into the tin, fold the cocoa powder into the remaining batter, then pour in over the vanilla layer. Use the handle of a spoon to swirl the batters together. Bake for about 1 hr or until the cake is risen and lightly browned and a skewer inserted into the middle comes out clean.

Remove the cake from the oven, let it cool for a while in the tin, then turn it out and leave to cool completely on a wire rack. Dust with icing sugar to serve.