Make the cookie dough: In a medium bowl, combine the flour, baking powder, baking soda, cornstarch, and salt and set aside.
In a large bowl or the bowl of a stand mixer, beat together the softened butter and sugars until pale and creamy.
Beat in the egg and vanilla until fully combined. Fold in the flour mixture just until combined. Fold in the chocolate chips.
Chill the dough in the fridge while preparing the brownie batter.
Make the brownie: Preheat the oven to 350°F (175°C). Grease an 8x13-inch sheet pan and line it with parchment paper.
Over medium heat or in the microwave, melt the butter and chopped chocolate, stirring often to combine. Set aside and let it cool.
Meanwhile, in a medium bowl, whisk together the eggs and sugar until pale and thick. Whisk in the cooled chocolate mixture and vanilla.
Fold in the flour and salt just until combined.
Pour half the brownie mixture into the prepared sheet pan. Top with spoonfuls of cookie dough, scattered in patches — it doesn’t need to cover the whole surface. Pour the remaining brownie batter evenly over the top. Scatter the remaining cookie dough in patches over the surface. Sprinkle with extra chocolate chips and flaky salt.
Cover the pan with aluminum foil and bake for 25 minutes. Uncover and bake for another 5–10 minutes, until lightly golden brown. Serve.