Marvellous Medicinal Marble Cake!

Both boys back at school at last! Only a small amount of shoving out the door was required this morning and the promise of something homemade to come back to.
So Chocolate Chip and Vanilla Marble Cake it is! I'll stick recipe at the bottom for anyone that wants it. It's a Mary Berry tray bake, simple but tasty and rather popular in this house.
S came home from school with scones made in HE, very tasty! Think he might take after his mum a wee bit.

Recipe
8 oz marg, 8oz caster sugar, 10oz self raising flour, 2tsp baking powder, 4 eggs, 2 tbsp milk, 1/2 tsp vanilla essence.
Mix all together. Spoon about half this mix into a lined 12 by 9 tin, dotting the spoonfuls apart.
Mix 1&1/2 tbsp cocoa powder & 2 tbsp hot water in a bowl and then add to remaining mix, stir in 2oz choc chips. Spoon this choc mixture in between the plain mix to fill the gaps.
Bake at 180/350 for 35 to 40 mins. Once cool drizzle with melted choc.