Preheat the oven to 160˚C/325/Gas Mark 3.
Line 2 baking trays with parchment paper.
In a large bowl, combine the peanut butter with the caster sugar until fully combined. Add the egg, baking powder and vanilla extract and stir well to combine. Roll 16 small balls and coat in some caster sugar.
Place on the prepared tray and gently press with a fork.
Bake in the oven for 10 minutes if making 16-18 small cookies or 14 minutes if making 8-12 cookies until golden brown. Remove from the oven and leave to settle before lifting from the tray.
Eat as needed or decorate a tart or dish with the broken pieces.