Pre-heat your oven to 180°c (160°c for fan assisted ovens or Gas Mark 4) and grease a couple of baking trays with a little butter.
In a large bowl cream the butter & icing sugar until light and fluffy.
Gradually add the flour & custard powder a little at a time until the mixture forms a paste.
Divide into small balls (about a teaspoon-tablespoon per biscuit) and place onto your pre-greased baking tray with a bit of space between. Flatten eat ball lightly with a fork.
Bake for 12-15 minutes until the appear dry on top.
Allow to cool on a wire rack. Mix the butter & icing sugar to form the butter cream and once the biscuits are completely cool, pipe or spread a little butter cream onto half the biscuits and sandwich together with the other halves.