Pre-heat your oven to 160ºC/140ºC fan.
Line your tins (2x 20cm sandwich tins) with liners or greaseproof paper.
Grate the milk chocolate (100g).
Put all of the ingredients into a large bowl (200g self-raising flour, 40g cocoa powder, 230g caster sugar, 4 large eggs, 230g soft margarine, ¼ tsp vanilla extract, 2 tsp milk and the grated chocolate) mix on a low speed until fully combined.
Divide the mixture evenly between the two tins and spread it out so it’s pretty even (it doesn’t have to be perfect as it’ll even out in the oven, but roughly flat is good).
Put the tins in the oven and cook for 30 minutes. After 30 minutes put a skewer in the middle and check if it comes out clean. If it does it’s done, if not pop it back in for a couple more minutes and check again.
Once cooked, remove the cakes from the oven and leave them to cool in the tins for 10 minutes.
After 10 minutes, remove the cakes from their tins and pop them onto a wire rack to cool completely before decorating.