Preheat the oven to 200°C/gas mark 6. Line a muffin tray with 12 cupcake cases. Place the sugar and spread in a large bowl and beat with a handheld electric beater until pale and fluffy. Add one egg to the mix at a time, beating after each addition.
Sieve the self-raising flour and cocoa powder into the mix and stir until combined. Add the milk to loosen the mix a little and stir again. Spoon into the prepared cake cups and bake for 12-15 minutes until firm and a skewer comes out clean. Transfer to a wire rack to cool.
In a large mixing bowl, use an electric hand whisk on a medium setting to beat the butter for 2-3 minutes, until pale and soft. Add half the icing sugar to the mixing bowl and beat on a medium setting at first, then on a higher setting until smooth. Repeat with the other half of the icing sugar and continue to beat for 1-2 minutes until the mixture is fluffy and pale. Add the vanilla extract and beat again.
When the cupcakes are completely cool, spread the buttercream over the tops to make smooth domes. Arrange 25-30 mini marshmallows on each cake, leaving enough space for an icing head.
Take a 10g piece of black icing and use your hands to roll and form it into the shape of a sheep's head. Use more black icing to make ears by rolling small sausages and pinching one end. Press into place, using a drop of water to stick if needed. Make the eyes by forming little balls of white icing, finishing with little balls of black icing for the pupils. Press on to the head, using a dot of water to help them stick if needed. Make two holes for the nostrils with the end of a cocktail stick and pop on top of the cake. Continue until all the sheep are made.