• Preheat oven to 350° F.
• Line an 8x8 baking dish with parchment paper and set aside.

For the Brown Butter

• Melt butter in a medium sauce pan on medium heat. Stir the butter to make sure it melts evenly. Once the butter is completely melted, it will start to foam up. Continue stirring the butter. As it cooks, it will start to change into a deeper color. This could take 6-8 minutes.

  You'll start to notice a nutty aroma and the butter will begin to have brown specs at the bottom of the pan. The entire process of browning butter from start to finish should not take more than 10 minutes on medium heat.
  10 tablespoon (140 g) unsalted butter
• Once butter is browned, pour it into a heat proof bowl and set it aside to cool for about 5 or 10 minutes.

For the Blondies

• Mix the batter: In a medium bowl, add cooled butter and sugar and mix using a spatula until combined. Add the egg, yolk, and vanilla and mix until combined. Add the salt and espresso powder and mix. Then add the flour and mix until incorporated. Lastly, add the chocolates and mix until completely combined.
  1 cup (200 g) brown sugar, 1 egg + 1 egg yolk, 2 teaspoon vanilla extract, ½ teaspoon salt, ¼ teaspoon espresso powder, 1¼ cup (150 g) all purpose flour, 3.5 oz (100 g) chocolate, chopped (chips works too), ⅓ cup (50 g) milk chocolate
• Bake the Blondies: Pour the batter into the prepared baking pan and bake for 20-25 minutes. You'll see the sides get a little light brown and the middle will have puffed up some. A toothpick inserted in the center should come out clean. Don't overbake.
• Remove from oven and allow to cool before slicing.