Line at least two baking sheets with baking parchment.
Sift the flour, cocoa powder and baking powder into a bowl and stir together.
Put the butter and sugar in a second bowl and whisk until pale and creamy.
Whisk in the egg, followed by the buttermilk and vanilla.
Sift in half the flour and cocoa mix and fold through, then sift in the other half and fold again to complete your batter. If the mixture is very loose, cover and place in the fridge to chill for 10 minutes.
Use a medium 5cm (2 inch) scoop to pick up level scoops of the batter and drop onto the lined trays. Allow space for them to spread while baking.
Bake for 12 minutes. Try not to have more than 2 trays in the oven at once as it can affect heat circulation.
Allow to cool on the tray for 5 minutes then transfer to a wire rack to cool completely.
Make the buttercream filling
Put the butter, vanilla and icing sugar in a clean bowl. Whisk until soft and creamy.
Transfer the buttercream to a piping bag and snip off the bottom 1cm.
Once the cake halves are totally cooled, flip half over to reveal the flat bottoms.
Transfer the buttercream to a piping bag and snip off the bottom 1cm.
Pipe a generous amount of buttercream onto the upturned halves.
Sandwich the remaining cake halves on top to complete your whoopie pies.
Video
Notes
Add the fine zest of 1 unwaxed orange to the buttercream to make them chocolate orange whoopie pies!