Preheat the oven to 180°C/gas mark 4. Line a 12-hole cupcake tin with cases.
Beat the butter with the sugar until light and fluffy, then whisk in the egg. In a separate bowl, mix together the flour and ground almonds.
Alternately mix the flour mixture and milk into the butter, egg and sugar mixture. Spoon into the cake cases. Bake for 20 minutes, or until a skewer inserted into them comes out clean. Leave to cool on a wire rack.
Meanwhile, gradually mix the icing sugar into the butter, then beat until light and creamy. Add the milk and melted chocolate and whisk into the buttercream.
Once the cakes are cool, cut out a round of sponge from the top, then cut that in half. Brush a little buttercream icing on to the top side of the 2 halves and dip in the sprinkles. Place 1 tsp buttercream in each cake, then place the sponge halves on top to create butterfly wings.