- STEP 1
Preheat the oven to 180C/Fan 160C/Gas mark 4 and line a deep 20cm loose-bottomed cake tin.
- STEP 2
Beat the butter and sugar together in a bowl then add the eggs one at a time, beating well after each addition. Fold through the flour, milk and vanilla extract (if you are using it) until the mixture is smooth.
- STEP 3
Put half the mixture into another bowl and add the cocoa powder and chocolate to one and the zest and food colour to the other. Dollop each mixture into the tin alternately.
- STEP 4
When all the mix has been used up, tap the bottom to make sure there are no air bubbles. Swirl a skewer or knife around the tin in a spiral motion a few times, but not so much that it gets mixed together.
- STEP 5
Bake in the oven for 45-55 minutes until a skewer inserted into the middle comes out clean. Turn out onto a rack and leave to cool then serve. Will keep for 3 days in an airtight container or freeze for up to 3 months.