Preheat your oven to 350°F (175°C) and prepare a 9x13-inch baking dish by greasing it or lining it with parchment paper.

In a medium saucepan over medium heat, melt 1 cup of unsalted butter, stirring frequently until browned.

In a large mixing bowl, combine the cooled brown butter with granulated sugar and light brown sugar, whisking until smooth.

Add 3 large eggs one at a time, mixing well after each addition, and stir in vanilla extract.

In a separate bowl, whisk together all-purpose flour, baking powder, baking soda, salt, and cocoa powder.

Gradually add the dry ingredients to the wet mixture, folding until just combined.

Fold in chocolate chips and chopped nuts into the batter gently.

Pour the batter into the prepared baking dish, spreading it evenly.

Bake for 25-30 minutes, checking for doneness with a toothpick.

Let them cool in the pan for 15-20 minutes before cutting into squares.