Preheat oven to 350°F (180°C)
Brown the butter and divide into 2 bowls, leaving one to cool
Add chopped chocolate to the hot butter and mix until melted
Add cocoa powder and vanilla extract to chocolate mixture and mix until fully combined
In a separate bowl, combine brown sugar and 3 eggs and whisk until light and fluffy (3-4 minutes by hand, 2 minutes with electric mixer)
Pour chocolate and brown butter mixture into egg mixture and mix until combined
Add flour and pinch of salt and fold with rubber spatula until mostly combined
Add chocolate chips and fold to evenly distribute
Once brown butter is cooled, combine with granulated sugar and brown sugar and mix
Add 2 eggs and mix for 1-2 minutes until mixture is paler and thicker
Add vanilla and mix to combine
Add dry ingredients (flour, baking powder, salt, espresso powder) and fold with spatula until almost all flour is mixed in
Add chocolate chips and mix until no flour remains
In a parchment-lined 8x8 pan, alternate transferring brownie and cookie doughs
Bake for 35-45 minutes, checking at 30 minutes with a knife until mostly clean
