Pre-heat your oven to 180°c (160°c for fan-assisted ovens, Gas Mark 4 or 350°F) and grease a couple of baking sheets with a little butter.
In a large bowl cream the butter/margarine & icing sugar until light and fluffy.
Gradually add the flour & custard powder a little at a time until the mixture forms a paste (ie. a soft dough)
Divide into small balls - about a teaspoon-tablespoon per biscuit - and place onto your pre-greased baking sheets with a bit of space between. Flatten each 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.
Custard Cream cookies are best enjoyed the day they are baked, but they can be stored in an airtight container for a few days. The texture may change over a few days - this is normal for home-baked biscuits.
If you want to freeze your Custard Creams, I recommend freezing the dough before baking. You can freeze the entire dough or divide it into batches. Remove the dough from the freezer and defrost it at room temperature before baking. Then bake and finish with buttercream as normal.
See the Free-from & Vegan box below for tips on how to make your Custard Creams vegan or free-from.