For the biscuits:
175g plain flour
3 tablespoons custard powder
3 tablespoons caster sugar
100g unsalted butter, cut into 1 cm cubes
1 egg
½ teaspoon vanilla extract
up to 1 tablespoon milk
For the filling:
100g icing sugar
1 tablespoon custard powder
50 g softened butter
1 teaspoon boiling water
To make the dough, pulse the flour, custard powder, baking powder and sugar together in a food processor until blended. Add the butter and pulse until the mixture resembles breadcrumbs. (If you don’t have a food processor, use your fingers, or a hand blender.)
Add the egg and vanilla extract and mix until the mixture comes together. If it’s still too dry, add the milk a drop at a time until it’s moist enough.
Make the custard cream filling:
Mix the icing sugar and custard powder in a bowl, then blend in the butter with an hand mixer. Add the boiling water and blend until smooth. Set aside until ready to use.
Preheat the oven to 180°C.
Roll the dough out on a well-floured surface to a thickness of about 4mm and prick over the surface with a fork. Cut out your biscuits and place on a parchment-lined baking sheet. Bake for 12-15 minutes until just golden at the edges, then remove to a rack to cool completely.
When the biscuits are cool, sandwich them together.
