Brookies: Brownie-Cookie Mash-Up Bars

Method

1. Preheat the oven to 350°F with a rack in the middle. Line a 13x9x2-inch (33x22x5-cm) baking pan with parchment paper, leaving an overhang on the sides. Grease the parchment lightly with butter or nonstick spray. If the parchment slips, dab a little butter or spray in the corners to hold it in place.
2. Make the brownie batter: In a medium microwave-safe bowl, melt the butter until bubbling. Add the chopped bittersweet chocolate and whisk until completely melted. If needed, microwave for 20 seconds more and whisk again. Alternatively, melt the butter and chocolate in a heatproof bowl over a pot of simmering water, stirring occasionally. Whisk in the granulated sugar, brown sugar, vanilla, and salt (the mixture will be slightly grainy). Whisk in the eggs, then the flour until just combined. Pour the batter into the prepared pan and spread evenly with a spatula. Set aside.
3. Make the cookie dough: In the bowl of a stand mixer fitted with the paddle attachment, beat the softened butter with both sugars for 3 minutes until light and fluffy. Scrape down the sides and bottom as needed. Add the vanilla and egg, and beat for 1 minute more. Scrape down the sides again. Add the salt, baking powder, and baking soda, and beat briefly until evenly combined. On low speed, mix in the flour. Finally, fold in the chocolate chips until just incorporated.
4. Drop small spoonfuls of the cookie dough evenly over the brownie batter (do not spread or press down). Cover the pan tightly with aluminum foil and bake for 20 minutes. Remove the foil and continue baking until the brownie edges look shiny and the cookie top is golden brown, about 20 more minutes. Transfer the pan to a wire rack and let cool completely.
5. To cut, lift the brookies out of the pan using the parchment overhang and place on a cutting board. Gently pull the parchment away from the edges. Using a sharp knife, cut into 2-inch (5-cm) squares directly on the parchment.