• Preheat the oven to 325°F. Line an 8 by 8-inch metal pan** with parchment paper, leaving an overhang. Spray with nonstick cooking spray.

Prepare the cookie layer:

• In a large microwave-safe bowl, heat the butter in 30-second increments until melted.
• Add the sugars to the hot butter and whisk to combine. Let cool until just warm.
• Add the egg and vanilla and whisk vigorously until smooth. Stir in the flour, salt, and baking powder with a rubber spatula. Gently fold in chocolate chips.
• Chill the dough in the fridge while you prepare the brownie layer.

Prepare the brownie layer:

• In a large microwave-safe bowl, heat the butter in 30-second increments until melted.
• Add the sugar to the hot butter and whisk to combine. Let cool until just warm.
• Whisk in the egg and egg yolk vigorously for 1 minute. Whisk in the cocoa powder, oil, and vanilla. With a rubber spatula, stir in the flour, cornstarch, and salt until just combined. Fold in chocolate chips.

Assemble:

• Scoop out nine 1-tablespoon portions of cookie dough and set them aside in a small bowl. Spread the remaining dough into the prepared pan in a thin, even layer. Don’t worry about making it perfect.
• Spread the brownie batter evenly over the cookie layer.
• Flatten the reserved cookie dough balls slightly with your palms, and place them on top of the brownie batter, letting the batter peek through.
• Bake at 325°F for 33-38 minutes, until the edges are set and the cookies have very lightly begun to brown. Bake 33 minutes for a gooey, fudgy center, or closer to 38 minutes for a chewier, more set texture.
• Top with flaky sea salt, then let cool in pan completely before removing and slicing.