Heat oven to 180C/160C fan/gas 4 and line a 12-hole muffin tin with deep muffin cases. Cream the butter with the sugar until pale and fluffy. Add the remaining cake ingredients and beat until smooth. Spoon into the muffin cases and bake for 20 mins or until golden and a skewer inserted into one of the middle cakes comes out clean. Cool for 5 mins in the tin, then completely on a wire rack.
Using a small, sharp serrated knife, cut a semi-circle piece of cake from the left and right of each cake, to make stepped edges, level with the cupcake case. Next, make a widthways cut about 3cm from the top of the cake, about 1cm deep. Slice a 5mm piece off the surface of the cake to meet this cut, to make a flat, raised face and prominent forehead. Chill for 10 mins to firm the crumbs.
Mix the icing sugar, milk and green colouring to make a very thick icing that flows slowly from the spoon. Spoon 1 tbsp onto a cake and let it begin to spread itself over the cut shape. Ease it here and there with a palette knife to coat. Add marshmallow neck bolts and eyes. Repeat for each cupcake. Leave to set, then pipe on the faces and hair.