Preheat the oven to 350ºF (180ºC) and line 2 large baking sheets with parchment paper (or use 1 baking sheet and clear it before starting the next batch).

Add the egg, sugar, vanilla (if using), and peanut butter to a large bowl.

Mix the ingredients together until a cohesive ball forms, scraping down the sides of the bowl as needed.

Use a medium cookie scoop or a heaping tablespoon to portion the dough.

Roll the balls in between your palms until smooth (they should be about 1 ½” in diameter).

Place on the prepared cookie sheet, spacing the balls about 2” apart (about 8-10 cookies on a conventional cookie sheet).

Press each cookie twice with a fork, creating a criss-cross pattern. If you have problems with the fork sticking, you may need to dip the fork into powdered sugar or use a new, clean fork.

Bake in the preheated oven for 10-13 minutes, or until the edges are lightly browned. The centers should look soft or undone.

Allow the cookies to cool for 5 minutes on the sheet before transferring them to a wire cooling rack.

Repeat with the remaining cookie dough on the other baking sheet (or use the baking sheet you just cleared off).

Cool cookies completely before transferring to storage.