Preheat the oven to 180C/350F/Gas mark 4
Cream together the butter and sugar. Add the flour and custard powder and mix thoroughly. Add the vanilla. Roll the dough into small balls and place them on a greased baking tray. Mark them with the back of a fork. Bake for 10 minutes. Cool on wire rack.
To make the filling, have the butter ready at room temperature. Cream all three ingredients together until well mixed. Sandwich the Cookies together with the filling.
