Preheat oven to 180ºC (160ºC fan oven, 350ºF, gas mark 4). Line 2 or 3 large baking trays with baking parchment.
First make the whoopie 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, bicarbonate of soda and salt into another bowl. Gradually add spoonfuls of the flour mixture along with the buttermilk, mixing well after each addition, to make a smooth, thick cake mixture.
Using a 5cm (2in) diameter ice cream scoop, drop 6 to 8 scoops of the mixture on to each baking tray, spaced well apart to allow for room for spreading. Bake in the oven for 12-14 mins until risen, slightly cracked and firm to the touch. Cool for 5 mins on the trays before transferring to a wire rack to cool completely.
For the topping: Sift the icing sugar and cocoa powder into a mixing bowl and gradually mix in 5-6tsp warm water to make a smooth, glossy spreadable icing. Spread thickly over the peaked side of half the cold pies, smoothing the icing with the back of a knife. Sprinkle with your favourite cake decorations and leave on a wire rack for a few mins 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. Add a few drops of vanilla to flavour. Either spoon the filling into a piping bag fitted with a plain ½cm (5mm) nozzle and pipe over the flat sides of the remaining pie halves, or alternatively, spread thickly using a knife.
To serve, when the iced pie tops are set, carefully sandwich them on to the filled pie halves and enjoy.