To make the icing
1. In a bowl beat butter until fluffy. Gradually add half of the icing sugar, beating well.
2. Slowly beat in the half of the milk and the vanilla. Slowly beat in remaining sugar. Beat in additional milk, if needed, to make of spreading consistency.
To make the fairy cakes
1. Preheat oven to 180 C / Gas mark 4.
2. Beat the butter and sugar until light and fluffy.
3. Mix in the eggs and vanilla.
4. Combine the flour, cocoa, baking powder and stir into the batter, than add the milk, just until blended. Spoon the batter into the prepared muffin tins.
5. Bake in the preheated oven for about, 20 to 25 minutes. Cool in the tin set over a wire rack.
6. When cool, ice with the buttercream icing.
7. Once all cakes are iced, draw a web using the tube of black decorating icing.
8. To decorate, you can place a plastic spider in the centre of each cake if desired.