My Mum is famous for these, so I want to preserve the recipe for prosperity:
6oz Butter
6oz Plain Flour
2oz Icing sugar
2oz Custard Powder
Cream butter, then add all dry ingredients.
Mix until it forms a cohesive dough; this will take a while. For 5 minutes you will be thinking "there's not enough butter!", but eventually it will come together.
Use a teaspoon to pick up small pieces of dough - about half a teaspoon full (they will expand by about a third on cooking).
Roll dough in your hand to make a smooth ball.
Use a fork to push ball down and make an indent on the back - but not too firmly.
Bake in a 180degree oven for 10-15minutes, or until they start to brown underneath. Remove from oven and cool on a rack.
Make icing - icing sugar, little knob of butter, hot water, and your choice of colourings or flavourings (half a passionfruit added to the mix makes a delicious icing).
Put a dob of icing on a biscuit and push another on top - but don't squeeze too hard.
This is why they are called YoYos.
Eat and enjoy!