• Preheat the oven to 350°F. Line an 18x13-inch half-sheet pan with parchment paper, leaving some overhang for easy removal. Lightly spray the parchment with nonstick cooking spray. Set aside.

Brownie Layer

• In a large microwave-safe bowl, combine 2 cups of the semisweet chocolate chips and the vegetable oil. Microwave in 30-second intervals, stirring after each, until the chocolate is completely melted and smooth. Set aside to cool slightly.
• In the bowl of a stand mixer fitted with the whisk attachment, add the granulated sugar, brown sugar, eggs, and vanilla. Mix until smooth and well combined.
• With the mixer on low, slowly pour in the slightly cooled chocolate mixture. Mix until fully combined.
• Add the flour, cocoa powder, and salt. Mix just until no dry streaks remain. Do not overmix.
• Fold in the remaining ½ cup semisweet chocolate chips.
• Spread the brownie batter evenly into the prepared pan.

Mint Chocolate Chip Cookie Layer

• In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and granulated sugar on medium speed until light and fluffy, about 2 to 3 minutes.
• Add the eggs and peppermint extract. Beat until well combined.
• Add the flour, baking powder, baking soda, cream of tartar, and salt. Mix just until combined.
• Fold in the 1 cup of whole chocolate chips and 1 cup of chopped chocolate chips.
• Drop spoonfuls of the cookie dough evenly over the brownie batter. Gently press and spread the dough to cover most of the brownie layer. It does not need to be perfectly even, as the cookie dough will spread as it bakes.
• Bake for 28 to 35 minutes, or until the cookie layer is lightly golden and a toothpick inserted into the center comes out with a few moist crumbs and no wet batter.
• Let the brookies cool completely in the pan before using the parchment overhang to lift them out. Cut into bars and serve.