Method

Preheat oven to 350°F (155°C Fan)

Sift flour, custard powder and cocoa into a large bowl.
Cream butter and sugar, then stir in the egg. Add the sifted ingredients from the other bowl and mix to an even dough.
Place onto worktop and using flour, roll out to about 1/8" thickness and cut into fingers.
Place on lined baking tray.
Bake for about 12 to 15 minutes, turning half way through baking time to assure even results.
Place on cooling rack and let cool before filling them.

Bourbon Biscuit Cream Filling
Mix all the ingredients together to form a stiff buttercream. Adjust with a few drops of liquid; just don't make the filling too soft.
Match up two biscuits at a time and sandwich together with the filling.
Continue until all the biscuits and filling are used.