Who says we’ve got to choose between brownies and cookies? Incredibly rich fudge brownies are baked atop a cakey peanut butter cookie base that is filled with crunchy peanuts. A peanut butter drizzle and more chocolate finishes these truly decadent bars.
Preheat the oven to 375 degrees F. Grease a 9-inch square pan with cooking spray, line with parchment paper, then spray the parchment.
For the peanut butter cookie base: Whisk together the flour, salt and baking soda in a medium bowl and set aside.
Melt the butter in a medium microwave-safe bowl and then stir in the brown sugar and peanut butter until smooth. Stir in the egg white and vanilla until fully combined. Stir in the flour mixture and peanuts until combined, then transfer the dough into the prepared pan.
Use a rubber spatula or your fingers to press the dough into an even layer. Bake until the cookie puffs up and the edges are light golden, 10 minutes.
Meanwhile, prepare the brownie batter.
For the brownie batter: Put the butter in a medium saucepan and cook over medium heat until melted and bubbling, about 1 minute. Stir constantly with a rubber spatula until the butter is light golden brown, about 2 minutes. Transfer the browned butter to a heat-resistant medium bowl and stir in the chocolate chips until smooth. Stir in the brown sugar, cocoa powder, hot water and salt until combined. Stir in the eggs, yolk and vanilla until the eggs are fully incorporated. Stir in the flour until completely combined and the batter is thick and shiny. Pour the batter over the just baked peanut butter cookie base and spread it with a rubber spatula or spoon until smooth.
Lower the oven to 300 degrees F and bake until a toothpick inserted about 3 inches from the edge of the pan comes out with fudgy crumbs, but no raw batter, 40 to 43 minutes. Let cool completely, about 2 hours 30 minutes.
To finish: Melt the peanut butter in a small microwave-safe bowl in 10 second intervals, stirring after each, until it has the consistency of a smooth sauce, about 20 to 25 seconds total. Transfer to a small resealable plastic bag, snip off the tip and drizzle the peanut butter sauce over the bars. Sprinkle with the mini chocolate chips and then slice into 16 squares. The bars can be stored in an airtight container at room temperature for up to 3 days.