Preheat oven to 350°. Toast ½ cup (71 g) roasted, salted peanuts on a rimmed baking sheet till browned and aromatic, 6–8 minutes. Let cool, then coarsely chop.
In the meantime, prepare dinner 6 Tbsp. unsalted butter in a medium skillet over medium warmth, swirling pan usually, till butter foams, then browns, 4–6 minutes. Switch brown butter with solids to a medium bowl; chill, stirring each 5 minutes, till cooled and starting to solidify, about quarter-hour.
Whisk ¾ cup (93 g) all-purpose 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. Utilizing an electrical mixer on excessive pace, beat 1 giant egg plus 1 giant egg yolk, ¾ cup (150 g) granulated sugar, ½ cup (170 g; packed) mild brown sugar, and ¼ cup water in a big bowl till combination is mild and falls again on itself in a slowly dissolving ribbon, about 2 minutes.
Scale back mixer pace to low and regularly add in dry components, then improve pace to excessive and beat 1 minute to develop gluten and hydrate flour. Add peanut butter, chopped peanuts, and brown butter and blend on low pace, scraping down sides of bowl as wanted, till absolutely included, about 30 seconds. Let dough relaxation at room temperature till barely firmed up, about 10 minutes.
Portion dough into 20 balls (about 2 heaping Tbsp. every) and switch to 2 parchment paper-lined rimmed baking sheets. Dip the tines of a fork in chilly water and flatten cookie dough balls to ½” thick, making a crosshatch sample on the highest of every. Chill, coated, 1 hour.
Organize racks in higher and decrease thirds of oven; place a dry, medium skillet on the underside rack, then improve oven temperature to 375°.
Sprinkle cookies with demerara or uncooked sugar and flaky sea salt. Working rapidly, switch 1 sheet of cookies to higher rack, then fastidiously pour 1 cup water into sizzling skillet; water will bubble and sizzle violently. Bake cookies till browned and edges are crisp, 12–quarter-hour. Let cool barely on baking sheet, then switch to a wire rack and funky fully. Repeat with second sheet.
Do forward: Cookie dough will be formed and frozen 1 month forward. Freeze in an hermetic container. Let relaxation at room temperature 1 hour earlier than baking.
Editor’s observe: This recipe was first printed in our March 2016 situation as a part of BA’s Finest, a group of our important recipes. Head this manner for extra of our greatest cookie recipes →