Preheat oven to 180 degrees C. Sift together the flour and custard powder then set aside.
Cream together butter, icing sugar and vanilla until fluffy. Stir in the flour and mix until well combined. Roll into teaspoon sized balls and place on a baking tray. Press down each lightly with the back of a fork. Bake in preheated oven for about 15 minutes then cool on a wire rack.
Cream together butter, icing sugar and custard powder. When biscuits have cooled, press together two biscuits with the filling between.