In a medium saucepan, melt the butter over medium heat.

Continue cooking, stirring frequently, until the butter foams, turns golden brown, and releases a nutty aroma (about 5-8 minutes).

Remove from heat and pour the browned butter into a large mixing bowl. Allow it to cool slightly (about 10 minutes).

Preheat your oven to 350°F (175°C).

Grease a 9x9-inch deep baking dish or line it with parchment paper, leaving an overhang for easy removal.

To the cooled brown butter, add brown sugar and granulated sugar. Whisk until smooth.

Add the eggs, egg yolk, and vanilla extract, and whisk until fully incorporated.

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

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

Fold in your nuts (if using), ensuring they're evenly distributed.

Spread the batter evenly into the prepared baking dish.

Bake in the preheated oven for 30-35 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs.

If you like a gooey center, bake closer to 30 minutes; for a firmer texture, bake closer to 35 minutes.

Let the blondies cool in the pan for at least 20 minutes before slicing.

Lift the blondies out using the parchment paper overhang and cut into squares or rectangles.