Preheat oven to 180C/350F/Gas 4 and line a baking sheet with parchment paper. Set aside.

Place the sugar, peanut butter, egg, and vanilla extract into a bowl and mix together to form a dough.

Scoop out the dough and roll into 1-inch balls.

Place the dough balls onto the prepared baking sheet and flatten gently with a spoon.

Cook for 10-12 minutes and remove from the oven and quickly add the Reese's Baking Mix to the hot cookies so that the candies stick.

Transfer to a wire rack to cool completely before serving.

Store for up to 3 days in an airtight container,