Sift flour and custard powder separately and set aside. Cream butter and sugar well
Add custard powder and mix, then add the flour a little at a time. You may need more or less. Mix well until it forms a soft, pliable dough
If you have a tool like the one I've used, you need to roll out the dough on a flat surface. Press the tool onto the dough so it forms the design on the dough. Use a small round cutter or the back of a piping nozzle and cut into rounds.
Place cookies on a lined baking tray and bake at 160 deg celcius for 15 to 20 minutes. Allow them to cool once removed from oven.
Beat the butter, icing sugar and vanilla until light and creamy. Add a little milk or fresh cream if icing is too thick. Sandwich the cookies together with the filling.