2Prepare the cookie layer: Mix flour, baking soda, and salt. Set aside. Combine melted butter, brown and granulated sugar, egg, and vanilla. Whisk until smooth and fluffy for about 2-3 minutes. Add dry ingredients and chocolate chips. Mix until just combined.
3Prepare the brownie layer: Melt butter in a saucepan over medium-low heat. Stir every 10 seconds until foamy (6-8 minutes). Continue until milk solids turn golden brown and smell nutty. Transfer to a heatproof bowl. Add cocoa powder, oil, vanilla, and chopped chocolate; mix until smooth. In a separate bowl Whisk 3 eggs and light brown sugar for 5-6 minutes until dissolved. Add powdered sugar; mix 1-2 minutes until combined. Add cooled browned butter mixture; mix until smooth. Add flour and chocolate chips; mix until well combined.
4Layer the cookie and brownie batters in the prepared pan.
5Bake for 50-55 minutes or until the edges are golden and the center is a little jiggly.
6Let cool completely before cutting into brookies.