• Preheat the oven to 350°F. Line an 18×13-inch half-sheet pan with parchment paper, leaving an overhang on the sides. Lightly spray the parchment with nonstick cooking spray.

Brownie Layer

• In a large microwave-safe bowl, combine 1 cup of the semisweet chocolate chips and the vegetable oil. Microwave in 20 to 30-second intervals, stirring after each, until the chocolate is melted and smooth. Set aside to cool slightly.
• In the bowl of a stand mixer fitted with the paddle attachment, beat together the granulated sugar, brown sugar, eggs, and vanilla on medium-high speed for 3 to 4 minutes, or until smooth and well combined.
• Reduce the mixer to low speed. Slowly pour in the cooled chocolate mixture and mix just until combined.
• In a separate bowl, whisk together the flour, cocoa powder, and kosher salt. Gradually add the dry ingredients to the wet ingredients, mixing on low speed just until combined.
• Fold in the remaining 1 ½ cups semisweet chocolate chips.
• Spread the brownie batter evenly into the prepared pan, smoothing the top with an offset spatula if needed.

Peanut Butter Cookie Layer

• Without washing the mixer bowl, cream together the butter, granulated sugar, and brown sugar until light and fluffy.
• Add the peanut butter, eggs, and vanilla. Mix until smooth.
• Add the flour, baking soda, and kosher salt. Mix on low speed just until combined.
• Drop spoonfuls of the peanut butter cookie dough over the brownie batter. Gently spread or press the dough into an even layer. It doesn't have to completely cover the brownie batter; a little brownie showing through creates a marbled look.
• Bake for 30 to 35 minutes, or until the cookie layer is lightly golden brown and a toothpick inserted into the center comes out with a few moist crumbs, but no wet batter.
• Let the brookies cool completely in the pan. Using the parchment overhang, lift them out of the pan and cut into bars.