Preheat your oven to gas mark 4, 180ºC, fan 160ºC.
Cream together the butter and sugar until they are fully mixed together.
Mix in the vanilla extract and milk.
Add the sieved flour and custard powder.
Mix this all together until the ingredients produce a smooth dough.
Wrap the dough in cling film and place into your fridge for about 20 mins to firm up – this makes it easier to cut the biscuits.
Roll out the dough until it is the thickness of a biscuit slice (I.e. one slice + cream filling + one slice = a single biscuit).
We found it easier to cut the dough in half here and then roll it out to cut the biscuit shapes.
Being food geeks, we had a custard cream mould at home – you can get these and others (e.g. Bourbon and Jammie Dodgers) for a few quid from eBay. They will make your biscuits look the real deal but they aren’t absolutely necessary.
Cut the dough into custard cream biscuit shapes and place them onto a baking tray lined with baking paper.
Bake in the oven, now keep an eye on them as they can burn quickly and you want to get them golden brown – this should take between five and ten minutes depending on the temperature of your oven.
Remove the biscuits from the baking tray and leave to cool on a wire rack.
Whilst the biscuits are cooling, you can make the filling.
Cream together the icing sugar, butter, custard powder and vanilla extract until smooth. Once the biscuits are cool, spoon a teaspoon of the filling onto a slice of biscuit and then top with another biscuit slice.
Repeat until all your biscuits are filled full of custardy tasting deliciousness and then serve. Keep the biscuits in an air-tight container, as they will go soft if you don’t, and they should keep for a few days.