Pre-heat oven to 180°C.
Beat the butter and sugar together until smooth and creamy. Add the vanilla essence.
Add the cocoa, flour and cornflour and mix into a biscuit dough.
Roll into small balls, place on a flat baking tray lined with baking paper.
Flatten with a fork and bake at 180°C for 20 minutes. Remove from the oven and cool.
Soften the butter, add the other ingredients and mix together thoroughly. When the biscuits are cold, ice the flat side with the filling and put them together.