Combine the butter, vanilla, icing sugar and custard powder in a large bowl. Mix well (using your hands) to make a dough.
Add in the flour and mix with your hands to combine.
Grab teaspoonfuls of the mixture and roll into balls.
Place on a lined baking tray. Press down with a fork.
Bake at 160º celsius (320 F) for 12 minutes.
Remove from the oven and cool completly.
Icing
Combine the icing sugar, butter, milk, boiling water and vanilla in a small bowl.
Mix until smooth, then add food colouring (optional).
Leave the icing to set for 5 minutes.
Smooth onto one biscuit, then sandwich on the second one.
Notes
These aren't freezer friendly. But they do keep for 3-5 days in the fridge. Why the fridge? Because the icing is only shelf stable for a short time due to the butter and milk. Change the flavour or colour of the icing:These are delicious with lemon essence, almond essence or even peppermint. You can also change the colour to your preferred icing colour.