You will need:
3 tbsp milk
Then you need to:
Per heat your oven to 180oC/160oC
Slowly melt the mars bars and the milk on a low heat until melted, although it doesn't have to be completely melted as a few gooey lumps are nice in the finished cupcake.
Beat the sugar and egg together until pale and creamy
Add half the flour and 2 of the eggs and mix well and then add the remaining flour and eggs and mix until combined.
Add the cocoa powder and melted mars bar mixture to the cake batter and mix well
Then divide the batter between your paper cake cases (it will make approx 20 cakes) and fill half way, I use an ice cream scoop to allow for equal distribution.
Cook for about 20 minutes - I check if mine are cooked by inserting a sharp knife to see if it comes out clean
Once cooled completely decorate with your chocolate buttercream
75g unsalted butter
2 tbsp cocoa powder
Beat all of the ingredients together until smooth and creamy and apply to your cupcakes
Then top your buttercream with chunks of mars bar