Cream butter and sugar until light and fluffy
Add eggs one at a time
Mix flour and baking powder. Add to the batter alternately with the milk, mixing just until combined
Remove about one third of the batter and stir in cocoa powder and milk
Layer light and dark batter into a well-greased bundt or loaf pan and gently marble with a fork
Bake at 350°F / 180°C for 50–55 minutes, until a toothpick comes out clean
Let cool briefly, turn out, and dust with powdered sugar
