When you can't decide between a cookie or a brownie, make a combo! Here, cookie dough is dotted over brownie batter and baked until the edges are cakey and the interior is fudgy.
Preheat oven to 350 degrees. Brush a 9-by-13-inch baking pan with butter; line with parchment, leaving a slight overhang on long sides. Whisk together flour, baking soda, baking powder, and salt.
In a large bowl, beat butter with both sugars on medium-high speed until light and fluffy, 6 minutes. Reduce speed to medium-low and beat in egg. Beat in vanilla. Reduce speed to low and add flour mixture; beat until just incorporated. Stir in chocolate.
Melt butter and chocolate in a medium heat-proof bowl set over (not in) a pot of simmering water, stirring until smooth. Remove from heat; whisk in granulated sugar. Whisk in eggs, one at a time, until combined. Whisk in cocoa and salt. Fold in flour until combined.
Pour brownie batter into prepared pan, smoothing top with an offset spatula. Crumble cookie dough evenly over batter. Cover with parchment-lined foil; bake until just set, 20 minutes. Remove foil and continue baking until golden brown and a toothpick inserted into center (avoiding chocolate chunks) comes out with moist crumbs, 27 to 30 minutes more. Let cool completely in pan on a wire rack. Lift brownies from pan using parchment; cut into 16 squares. Brownies can be stored in an airtight container at room temperature up to 3 days.