Pre-heat your oven to 160ºC/140ºC fan.
Put your cupcake cases into a muffin tin.
Grate the milk chocolate (75g).
Put all of the ingredients into a large bowl (150g self-raising flour, 30g cocoa powder, 175g caster sugar, 3 large eggs, 175g soft margarine, ⅛ tsp vanilla extract, ½ tbsp milk and the grated chocolate) mix on a low speed until fully combined.
Divide the mixture evenly between the 12 cupcake cases (I use a cupcake scoop to help split it evenly).
Put the tin into the oven and cook the cupcakes for 20-25 minutes. After 20 minutes put a skewer in the middle and check if it comes out clean. If it does they're done, if not pop them back in for a couple more minutes and check again.
Once cooked, remove the cupcakes from the oven and leave them to cool in the tins for a few minutes.
Remove the cupcakes from their tin and pop them onto a wire rack to cool completely.