Preheat the oven to gas 4 / 180C / 350°F and line a baking sheet with parchment paper
Cream the margarine and sugar together
Stir in the flour, custard powder, baking powder and vanilla until you form a dough
Roll into balls and place on the baking sheet. You'll get around 24 x 1 inch sized balls
Flatten with the back of a fork
Bake for 12 -14 minutes then let cool on a wire rack
To make the filling, cream the margarine until soft then beat in the sugar, custard powder and vanilla
Spread on one cookie then sandwich another on top, repeating for all
Custard powder is simply cornstarch with a touch of yellow colouring and a little almond or vanilla flavouring. If you can't find custard powder in the shops just use the same quantity of cornstarch and add an extra 1/2 tsp vanilla
