Cream together the vegan butter and caster sugar until it's nice and smooth.
Sieve the flour and custard powder into the bowl with the creamed butter and sugar. (You can skip sieving the flour if you wish but don't be tempted to skip sieving the custard powder or you'll have hard lumps in your biscuits).
Add the vanilla extract and stir everything together with a wooden spoon until it comes together in a soft dough. If it's too soft to work with then refrigerate before the next stage.
Flour up your countertop and rolling pin and roll out your dough to about 5mm thickness. Cut out your chosen shapes making sure you have an even number.
Put the shapes onto baking sheets covered with baking parchment. Pop them in the fridge for at least 30 minutes to firm up--it helps them to keep their shape as they cook.
While the biscuits are chilling you can make the custard cream filling. Put the icing sugar into a bowl and sieve the custard powder over the top. Add the vegan butter and mix until you have a soft spread.
If you're going to pipe your filling onto the biscuits you can fill your piping bag now while the mix is pretty loose and then put the piping bag into the fridge. If you're spooning on your filling then put the bowl into the fridge.
Heat your oven to 180C/350F. When the oven is up to temperature and your biscuits have chilled, put the baking sheets into the oven for 10 minutes. The biscuits are done when they are slightly golden on the edges. We're looking for a mostly pale finish. Leave to cool.
Once the biscuits have cooled (it won't take long) you can begin to add the filling. The recipe here makes quite a lot of filling--if you put in the amount you get in storebought Custard Creams then you'll have a lot leftover, so be as generous as you like. Pipe or spoon your chosen amount onto the smooth side of a biscuit and place another biscuit on the top. Be fairly gentle but the biscuits are quite sturdy.
Store in an airtight container and enjoy.