Preheat your oven to 350°F (175°C) and line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal.
In a medium saucepan over medium heat, melt the butter. Continue cooking, stirring frequently, until the butter turns golden brown and starts to smell nutty (about 5-7 minutes). Be careful not to burn it.
Remove the saucepan from heat and transfer the browned butter to a mixing bowl. Let it cool for a few minutes.
Once cooled, add the brown sugar and granulated sugar to the bowl, stirring until well combined.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, mixing until just combined (do not overmix).
Fold in the chocolate chips and nuts if using.
Pour the batter into the prepared baking pan and spread it evenly.
Bake for 20-25 minutes, or until the edges are lightly golden and a toothpick inserted in the center comes out with a few moist crumbs.
Allow the blondies to cool in the pan for about 10 minutes before lifting them out using the parchment paper. Let them cool completely on a wire rack before cutting into squares.
Notes
For extra flavor, try adding different types of nuts or chocolate chips.