Sift the flour, cocoa powder and baking powder into a bowl. Stir together.
Cream together the butter and sugar, then beat in the egg until fully incorporated. Mix in the buttermilk and vanilla.
Slowly add the sifted dry ingredients to the wet ingredients in two additions (I folded in with a metal spoon at the stage, rather than beating in).
Chill the mixture for 30 minutes. (I used chilled buttermilk and found the mixture was already very stiff so I moved straight on).
Once chilled, use a piping bag fitted with a 1cm (1/2 inch) nozzle to pipe circles of mixture onto the baking tray, leaving a few centimetres between each one. (The book doesn’t say how big to make them, so I drew 2 inch circles with pencil on to the paper and filled those).
Bake for 10 minutes, and then allow to cool completely on a wire rack.
While they are cooling, beat together the butter and icing sugar in a bowl until soft a creamy. Add the orange zest and mix until fully incorporated.
Once the cake halves are cooled, spread or pipe a generous amount of the orange buttercream onto one half and sandwich it together with another half. Repeat this until you have filled all of your whoopie pies.