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

Place unsalted butter in a medium saucepan over medium heat. Stir frequently until the butter turns golden brown and has a nutty aroma, about 5-7 minutes.

In a large mixing bowl, combine granulated sugar, light brown sugar, and the cooled brown butter. Whisk until smooth with no lumps.

Add eggs one at a time while mixing well after each addition. Stir in the vanilla extract until well incorporated.

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

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

Gently fold in semi-sweet chocolate chips and chopped nuts if using.

Transfer the batter into the prepared baking dish, smoothing the top for an even layer.

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

Let the brookies cool for about 10 minutes, then slice into squares.