FOR THE BISCUITS:
1 In a large bowl, cream together the butter and sugar for about two minutes until very pale and soft. Add the vanilla extract, mix well, then sift in the flour, custard powder and salt and continue to mix until well combined.

2 Add two tablespoons of the milk, then add another tablespoon, if needed,  to bring the dough together.

3 Tip the dough out on to the work surface, wrap in clingfilm and chill for at least 30 minutes.

4 Preheat the oven to 180 degrees (160 degrees fan). Line two baking trays with parchment paper.

5 Dust the work surface with a little flour and roll out the biscuit dough into a large rectangle, turning the dough with each roll to prevent it sticking to the surface, until the dough is about 3-4mm thick. If using an embossed rolling pin, roll over the dough once at this point to create a pattern on the dough.

6 Cut out 6cm circles using a fluted cookie cutter or any shape cutter preferred. Re-roll the scraps until you have used up all the dough.

7 Place the biscuits on the baking trays, leaving a centimetre or two between each and bake in the preheated oven for about 10-12 minutes, until pale golden.

8 Remove from the oven and leave to cool completely.

FOR THE FILLING:
1 Put the butter in a large mixing bowl and sift in half of the icing sugar. Cream the butter and sugar together for about two to three minutes until pale and creamy.

2 Sift in the remaining icing sugar with the custard powder and beat again until smooth. The buttercream will be quite thick.

3 Add the vanilla extract, followed by a tablespoon of milk and beat until soft and smooth. Add the remaining tablespoon of milk if the buttercream is a little stiff. Set aside at room temperature until ready to assemble the biscuits.

TO ASSEMBLE:
1 Place the buttercream in a piping bag and pipe about a teaspoon on to half of the biscuits. Sandwich the custard creams together using the remaining biscuits.