Preheat the oven to 180C/350F/Gas 4, and line a baking tray with parchment paper or a silicone mat. Set aside.

In a large mixing bowl, mix together the peanut butter, sugar, egg, and vanilla until completely combined and a big ball of dough is formed.

Using a small cookie scoop (about 1oz per ball), scoop the dough onto the prepared baking tray. Dip a fork in sugar, and lightly flatten each cookie, making a criss-cross pattern.

Bake for 10 - 12 minutes, or until the edges are lightly browned. The cookies will be super soft, but they'll firm up as they cool.

Allow to cool for 10 minutes before transferring them to a wire rack to cool completely.