Preheat the oven to 350°F (180°C or 170°C for a fan oven).

Grease and flour or line a 9 x 5 x 2½ inch (2 lb) loaf pan.

Cream the butter in an electric mixer.

Add the sugar and cream until light and fluffy.

Beat in the eggs.

Sift the flour, baking powder and salt together and stir into the batter.

Divide the batter in half, placing each half in a separate bowl.

Beat the milk into the first bowl of batter. Then sift in the cocoa powder and beat that in too.

To the second bowl, add the orange juice and zest. Beat thoroughly.

Alternate spoonfuls of the mixture in the loaf pan.

Drag a knife or skewer through the mixture in swirls to create a marble effect. Carefully smooth the top of the batter, without disturbing the marbling.

Bake for 45 to 55 minutes until a skewer inserted in the centre of the cake comes out clean.

Cool the cake in the tin for ten minutes and then turn out and cool on a rack until completely cool.

Melt the dark chocolate in a double boiler and drizzle over the cake. Allow the chocolate glaze to set.

Put the icing sugar in a bowl and gradually add orange juice until a thick but drizzle-able consistency is reached. (You will need about 1 to 2 tablespoons of juice.)

Drizzle the orange glaze over the cake. Allow the glaze to set.

Cut the cake in slices to serve.