Prep: 1 hour. Bake: 10–11 minutes. Cool: 1 hour. Decorate: 30 minutes. Lasts: 3+ days, at room temperature.

In a large bowl, beat the butter and sugar together well. Add the vanilla extract, plain flour and instant custard powder and start to beat together, adding the milk slowly so that the dough combines. Roughly shape the dough into a rectangle, wrap with clingfilm and chill for 30 minutes. Towards the end of the 30 minutes, preheat the oven to 180ºC/160ºC fan and line 2–3 large baking trays with parchment paper.

Place the chilled dough on a lightly floured work surface. Roll the dough into a large rectangle about ½cm thick. Cut the dough into 50 small rectangles with a cutter measuring about 5 x 2.5cm. Carefully place each biscuit onto the lined trays, spaced slightly apart. Bake in the oven for 10–11 minutes until golden. Leave to cool on the trays.

Filling: In a large bowl, beat the butter for several minutes to loosen it. Add the icing sugar and instant custard powder and beat together again. Add the vanilla extract and 1 tablespoon of milk at a time until you reach a smooth and pipeable consistency. Transfer to a piping bag with the piping nozzle of your choice fitted and pipe a small amount of custard buttercream onto half the biscuits. Sandwich each one with another biscuit and enjoy!

Notes:

• You can shape the biscuits however you like – I made rectangles as I wanted a classic shape, but you can make squares, circles, or even use a biscuit cutter to make random shapes like stars.

• You can add more flavour to these biscuits if you want, but I like them as they are. One teaspoon of almond extract is a delicious addition.