Heat oven to 190C/170C fan/gas mark 5. Grease a loaf tin and put a rectangle of baking parchment in the bottom. Cream the margarine and sugar together, then add eggs one at a time, then fold the flour in and add vanilla essence. Mix until smooth.
Divide the mixture between two bowls. Mix milk and cocoa powder until a paste is formed. Add cocoa paste to one of the bowls, along with the grated chocolate and crush the flake bars in and mix well.
Take two spoons and use them to put a dollop of each mixture in alternating each time. When you have used all the mixture tap the tin on the work surface to remove any air bubbles. Then take a skewer and gently swirl it around to create the marble effect.
Bake the cake for 45 minutes or until a skewer/knife can be inserted and come out clean. Turn on to cooling rack when cooked and then serve with double cream. The cake will keep for up to 4 days and can be frozen for up to 30 days.