Preheat oven to 180°C (160°C fan oven, Gas 4). Line 3 large baking trays with baking paper. First make the pies. In a mixing bowl, cream the butter or margarine and sugar together until pale and creamy then beat in the egg.
Sift the flour, cocoa and bicarbonate of soda into another bowl. Gradually add spoonfuls of the flour mixture to the creamed mixture, stirring well after each addition, along with sufficient buttermilk to make a smooth, thick cake mixture – Note: you may not require the full 250ml buttermilk.
Using a teaspoon, drop walnut sized scoops of the mixture on to baking trays, spaced well apart to allow room for spreading. Bake in the oven for 10-12 minutes until risen and firm to the touch. Cool for 5 minutes on the trays before transferring to a wire rack to cool completely.
For the topping, sift the icing sugar into a mixing bowl and add the red and yellow food colouring. Gradually add 5 to 6 tsp warm water to make a smooth, spreadable icing. Spread thickly over the peaked side of half the cold cakes. Sprinkle with chocolate stars, moons & hearts and leave to set.
For the filling, put the butter in a mixing bowl and beat until soft. Gradually sieve in the icing sugar, beating well after each addition, to make a smooth, spreadable icing. Stir in the red and yellow food colouring and spread thickly over the flat sides of the remaining pie halves using a knife.
To serve, when the iced pie tops have set, carefully sandwich them on to the filled pie halves.