Heat oven to 180C/fan 160C/gas 4. Grease and line the base of a loaf tin (8 x 21cm/ 2lb) with baking parchment. Beat the butter, sugar, eggs and flour together in a large bowl with an electric whisk or in a food processor until lump free.
Split the mixture into two bowls, beat the milk, followed by the cocoa powder into one. Beat the orange juice, zest and orange food colouring, if using, into the other.
Spoon alternate dollops of the mixture into the cake tin, then use a skewer to create a marble pattern by dragging it through the mixture in swirls. Make sure you don’t overmix or you won’t see the pattern. Smooth the surface if necessary.
Bake the cake for 45 - 55 mins until golden and risen, and a skewer poked in comes out clean.
Leave the cake in the tin to cool, then turn out. Melt the chocolate in a bowl over barely simmering water or gently in the microwave. Use a spoon to drizzle the chocolate over the cake.