Cream butter and sugar. Add milk. Add custard powder, flour and essence. Form into a dough. Roll it out thinly, prick all over and cut into small rounds. Bake in a greased tin, in a moderate hot oven for about 15 minutes, until very lightly colored. Cook on a wire tray. When biscuits are cool, sandwich together in pairs with butter icing, either plain or flavored with orange, chocolate or vanilla. For butter icing, cream butter, add sieved icing sugar little by little. Beat till smooth and creamy. Add essence.