1.Preheat oven to 350 F. Line an 8*8 inch pan with parchment paper.
2. Prepare 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 min. Add dry ingredients and chocolate chips. Mix until just combined.
3. Prepare 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.
4.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.
5.Layer the cookie and brownie batters in the prepared pan.
6.Bake for 50-55 minutes or until the edges are golden and the center is a little fudgy.
7.Let cool completely before cutting into brookies.
8.Note: You can adjust the layering to your preference, or by creating a marbled effect by swirling the two batters together.