Preheat the oven to 180C/350F/Gas 4, and line a baking tray with parchment paper or a silicone mat. Set aside.
Stir together the peanut butter, sugar, egg, and vanilla extract until combined.
Using a small or medium scoop, scoop the dough onto the prepared baking tray. Dip a fork in sugar, and 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.
Cookies stay fresh, kept in an airtight container, at room temperature for up to 1 week. Cookie dough balls can be kept in the fridge for up to 5 days, or frozen for up to 3 months. Bake from frozen for an extra minute.
The calories are an estimate only.