Preheat oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a small bowl, whisk together flour and baking soda. Set aside.
In a medium bowl, cream butter, peanut butter, brown sugar, and granulated sugar until smooth. Stir in egg yolk and vanilla until well combined.
Add flour mixture to the wet ingredients and mix until a soft dough forms.
Chill dough in the freezer for 10 minutes (or fridge for 30 minutes) to prevent spreading.
Scoop dough into 8 equal portions (~1 tablespoon each). Roll into balls and place 2 inches apart on the baking sheet.
Use a fork to press crisscross patterns on top of each dough ball.
Bake for 12–14 minutes, until edges are set and lightly golden. Centers may look slightly underbaked but will firm up as they cool.
Cool on the baking sheet for 5 minutes, then transfer to a wire rack.
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Notes
After adding the flour, mix just until you can't see it anymore. Overmixing can lead to tougher cookies.If you have the time, chill the dough for at least an hour, or even overnight. This helps the flavors to meld together and also prevents the cookies from spreading too much during baking.Using brown sugar instead of white sugar gives the cookies a deeper, more caramel-like flavor and a softer, chewier texture.Peanut butter cookies can go from perfect to burnt very quickly. They should be slightly underbaked when they come out of the oven; they will firm up as they cool.Peanut butter cookie dough can be dense, so make sure to flatten the cookies slightly with a fork before baking. This ensures that they cook evenly.