For the best flavour, use a good-quality peanut butter with no added sugar. The crunchy coating is made by rolling the cookie mixture in roasted (but unsalted) peanuts before baking.
(makes about 20)
115g unsalted butter, softened
125g crunchy peanut butter
140g light muscovado sugar
1 large egg, lightly beaten
1/2tsp real vanilla essence
225g self-raising flour
200g roasted unsalted peanut halves
several baking trays, greased
Put the soft butter, peanut butter, sugar, beaten egg, vanilla and flour in a large bowl. Mix well with a wooden spoon.
When thoroughly combined, take walnut-sized portions of the dough (about a tablespoon) and roll into balls with your hands. Put the peanut halves in a shallow dish, then roll the dough in the nuts. Arrange the balls well apart on the prepared trays, then gently flatten slightly with your fingers.
Bake in a preheated oven at 180C/350F/Gas 4 for 12-15 minutes until light golden brown.
Let cool on the trays for a couple of minutes to firm up, then transfer to a wire rack to cool completely.
Store in an airtight container and eat within five days or freeze for up to a month.