In a medium saucepan, melt the butter over medium heat.
Cook, stirring frequently, until the butter turns golden brown and smells nutty, about 5–7 minutes.
Remove from heat and let cool slightly.
Make the Dough:
In a large mixing bowl, whisk together the browned butter, light brown sugar, and granulated sugar until smooth. Add the eggs and vanilla extract, mixing until fully combined.
Fold in the all-purpose flour, kosher salt, and baking powder until just combined. Avoid overmixing.
Gently fold in the chocolate chips and chopped nuts, if desired.
Bake:
Line a 9x13-inch baking pan with parchment paper, leaving an overhang for easy removal.
Spread the blondie batter evenly into the prepared pan, smoothing the top.
Heat oven to 325°Fahrenheit (163°Celsius).
Bake for 35–40 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs.
Cool completely in the baking pan before using the parchment paper to lift out the blondies. Slice into 24 squares.
Notes
Achieve the Perfect Crinkly Top: Beat the sugars into the warm browned butter until smooth and glossy before adding the eggs.
Prevent Sinking: Spread the batter evenly in the prepared pan and tap it on the counter a few times to release any air bubbles.
Test for Doneness: Use a toothpick; it should come out with moist crumbs but no raw batter.
Cool Completely: Let the blondies cool in the pan before slicing for clean edges.