Preheat the oven to 180C gas mark 4 and line two baking trays with baking parchment.
Melt the butter and chocolate in a heatproof bowl over a pan of barely simmering water, making sure the bowl doesn't touch the water.
Remove from the heat and allow to cool slightly.
Whisk the sugar, eggs and vanilla in a separate bowl for about 3 minutes or until light, fluffy and pale in colour, then fold in the chocolate mixture.
Sift the flour, cocoa and baking powder together and fold into the mixture. Place tablespoons of the mixture on to the baking trays (the mixture should make about 20 biscuits) and bake for 10 to 12 minutes.
Remove from the oven and allow to cool.
For the filling, cream the ingredients together (initially with a wooden spoon - if you start with an electric whisk you may disappear in a cloud of icing sugar). When the pies have cooled, pair them up and apply a layer of filling to one of the flatter sides and sandwich them together.
TipFor those who would like to try something more akin to the original, make a hot meringue from either a chocolate meringue pie or a baked Alaska recipe and use that as a filling.