Preheat oven to 350°. Toast ½ cup (71 g) roasted, salted peanuts on a rimmed baking sheet except browned and fragrant, 6–8 minutes. Let wintry, then coarsely cleave.
In the intervening time, cook 6 Tbsp. unsalted butter in a medium skillet over medium heat, swirling pan continually, except butter foams, then browns, 4–6 minutes. Transfer brown butter with solids to a medium bowl; relax, stirring each and each 5 minutes, except cooled and starting up to solidify, about quarter-hour.
Toddle ¾ cup (93 g) all-cause flour, ¾ tsp. Diamond Crystal or ½ tsp. Morton kosher salt, ½ tsp. baking powder, and ¼ tsp. baking soda in one other medium bowl. Using an electric mixer on high velocity, beat 1 good egg plus 1 good egg yolk, ¾ cup (150 g) granulated sugar, ½ cup (170 g; packed) gentle brown sugar, and ¼ cup water in a good bowl except mixture is gentle and falls attend on itself in a slowly dissolving ribbon, about 2 minutes.
Reduce again mixer velocity to low and step by step add in dry substances, then amplify velocity to high and beat 1 minute to manufacture gluten and hydrate flour. Add peanut butter, chopped peanuts, and brown butter and mix on low velocity, scraping down sides of bowl as wished, except fully integrated, about 30 seconds. Let dough leisure at room temperature except moderately firmed up, about 10 minutes.
Piece dough into 20 balls (about 2 heaping Tbsp. each and each) and switch to 2 parchment paper-lined rimmed baking sheets. Dip the tines of a fork in wintry water and flatten cookie dough balls to ½” thick, making a crosshatch pattern on the discontinuance of every and each. Kick again, covered, 1 hour.
Tackle racks in upper and lower thirds of oven; pickle a dry, medium skillet on the underside rack, then amplify oven temperature to 375°.
Sprinkle cookies with demerara or uncooked sugar and flaky sea salt. Working fleet, switch 1 sheet of cookies to upper rack, then fastidiously pour 1 cup water into hot skillet; water will bubble and sizzle violently. Bake cookies except browned and edges are crisp, 12–quarter-hour. Let wintry moderately on baking sheet, then switch to a wire rack and wintry fully. Repeat with 2d sheet.
Kind forward: Cookie dough will also be shaped and frozen 1 month forward. Freeze in an airtight container. Let leisure at room temperature 1 hour sooner than baking.
Editor’s repeat: This recipe used to be first printed in our March 2016 enlighten as phase of BA’s Simplest, a series of our crucial recipes. Head this form for more of our most intriguing cookie recipes →