1 ¼ cups unbleached all-purpose flour (6 1/4 ounces)
¼ teaspoon baking soda
¼ teaspoon table salt
4 tablespoons unsalted butter, melted and cooled
1 large egg
2 teaspoons vanilla extract
1 cup packed dark brown sugar (7 ounces)
2 tablespoons chunky peanut butter
¼ cup salted dry-roasted peanuts, chopped coarse
1 Adjust an oven rack to the middle position and heat the oven to 350 degrees. Line 2 baking sheets with parchment paper.
2 Whisk the flour, baking soda, and salt together in a medium bowl; set aside. In a large bowl, whisk the butter, egg, and vanilla together. Stir in the brown sugar and peanut butter until smooth, smearing any remaining clumps against the side of the bowl with a rubber spatula. Stir in the flour mixture and peanuts until thoroughly combined.
3 Working with a level tablespoon of dough each time, roll the dough into 1-inch balls. (If the dough is too soft to roll, refrigerate it until firm.) Place the balls on the prepared baking sheets, spacing them about 2 1/2 inches apart (you will fit 12 cookies on each baking sheet).
4 Bake the cookies, one tray at a time, until the edges are light golden and the centers are just set, 11 to 13 minutes, rotating the tray halfway through baking (do not overbake). Let the cookies cool on the baking sheets for 5 minutes, then serve warm, or transfer to a wire rack and let cool completely. Bake the second tray while the first tray cools.