Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan or line it with parchment paper for easy removal.
In a medium saucepan over medium heat, melt the butter, stirring frequently until it turns golden brown and develops a nutty aroma. This should take about 5-7 minutes. Once browned, remove from heat and let it cool slightly.
In a large mixing bowl, combine the brown butter, brown sugar, and granulated sugar. Whisk together until well combined.
Beat in the eggs, one at a time, mixing well after each addition. Then stir in the vanilla extract until fully incorporated.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
Gradually add the dry ingredient mixture to the wet ingredients, stirring gently until just combined. Be careful not to overmix.
Gently fold in the chocolate chips and nuts (if using) until evenly distributed throughout the batter.
Pour the batter into the prepared baking pan, smoothing the top with a spatula. Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Allow the blondies to cool in the pan for at least 10 minutes before transferring to a wire rack to cool completely. Once cooled, cut into squares.
Notes
Serve warm with a sprinkle of sea salt or drizzle with melted chocolate for decoration.