- 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.