Whisk melted butter, light brown sugar, and granulated sugar in a large bowl until combined.
Whisk in egg and vanilla extract vigorously until creamy and lighter in color.
Add baking powder, kosher salt, and cornstarch and mix evenly.
Fold in all-purpose flour gradually using a spatula, adding 1 cup of chocolate chips halfway through to form thick, sticky dough.
Refrigerate the cookie dough uncovered while preparing the brownie batter.
Melt butter and coconut oil in a medium saucepan over medium heat.
Whisk in cocoa powder and let bubble for 30 seconds, then remove from heat and stir in dark chocolate chips until smooth.
In a large bowl, whisk granulated sugar, light brown sugar, and eggs vigorously until creamy and lighter in color.
Add vanilla extract and kosher salt, then whisk in the chocolate mixture.
Stir in all-purpose flour just until combined.
Grease and line a 9×13 baking pan with parchment paper, leaving overhangs.
Spread brownie batter evenly in the pan and chill in the refrigerator for 15 minutes while preheating oven to 350°F.
Press spoonfuls of chilled cookie dough flat and arrange them over the brownie layer to cover the surface.
Fill gaps with remaining cookie dough to fully cover the brownie.
Sprinkle remaining chocolate chips on top and gently press into the surface.
Bake for 30 to 35 minutes until bars are puffed and lightly golden.
Drop the baking pan gently a few times after removing from oven to deflate the bars, then sprinkle with flaky sea salt.
Cool at room temperature for 20 minutes, then refrigerate for at least 1 hour until just warm.
Lift out the bars using the parchment overhang and slice into 12 large or 24 smaller squares. Run a knife under the bars to help release if needed.
Enjoy your decadent brookies!