Preheat oven to 350°F. Line baking sheet with parchment paper.
Mix peanut butter, sugar, and egg together in a bowl using an electric mixer or spoon, until smooth and creamy. Roll mixture into small balls and arrange on a baking sheet; flatten each with the back of a fork, making a criss-cross pattern.
Bake in the preheated oven for 8-10 minutes. Cool cookies on the baking sheet for 2 minutes before moving to a cooling rack for a few more minutes.