Combine the dry ingredients. In a large mixing bowl, whisk together 2½ cups (300g) all-purpose flour, ½ cup (50g) unsweetened cocoa powder, 1 tsp espresso powder, 2 tsp baking powder, 1 tsp baking soda, 1 tsp cornstarch, and 1 tsp kosher salt.
Cream butter and sugar. In the bowl of a stand mixer, combine 1 cup (227g) cold, cubed unsalted butter, 1 cup (200g) light brown sugar, and ½ cup (100g) granulated sugar. Beat on medium speed until no large pieces remain, about 2 minutes. Add 1 whole egg, 2 egg yolks, and 1 tsp vanilla bean paste, then continue beating until well mixed, about 30 seconds on medium.
Add the dry ingredients. With the mixer off, add ⅓ of the dry ingredients, then mix on low speed until just barely combined. Watch out for flying flour. Repeat two more times with the remaining dry ingredients.
Add peanut butter chips. Now for the best part: pour an entire 10oz (283g) bag of Reese’s peanut butter chips into the batter. Mix on low speed until they’re fully incorporated into the batter, then stop – do not overmix!
Portion the cookies. Line a rimmed baking sheet with parchment paper, then use a scale to weigh the dough into approximately 5½ oz portions – each portion should weigh 155g each. The dough rounds should be taller than they are wide. Do not smooth them out into perfectly round balls – keep them rustic and craggly. Also, they’re meant to be very large! You can keep them all on one sheet pan now, but when you go to bake them, you’ll want no more than 4 per baking sheet.
Chill. Place the cookie dough in the fridge to chill for at least 30 minutes, but longer (even overnight) is totally fine.
Bake. Preheat an oven to 375°F (191°C). Transfer 4 of the chilled cookies to a parchment lined baking sheet, then bake straight from the fridge for 12-15 minutes, or until the tops are golden brown. As with all cookies, they should look slightly underbaked (they’ll continue to firm up as they cool). Immediately sprinkle each with a little flaky sea salt. Cool for about 10 minutes on the sheet pan before transferring to a wire rack. They need this time to set. Repeat with remaining cookie dough (just keep it in the fridge until ready to bake). Enjoy warm or at room temperature!