Preheat the oven to 350℉/175℃ and line an 8x8 baking pan with parchment paper or foil.

Melt the butter in a small saucepan over medium-low heat until it begins to foam and brown.

Pour the brown butter into a large microwave-safe bowl and let cool slightly.

Add the brown sugar and granulated sugar to the bowl, whisk until smooth, and microwave for 20-30 seconds.

Whisk again for 1-2 minutes.

Whisk in the salt, vanilla extract, and cocoa powder. Microwave for another 20-30 seconds and whisk for about a minute.

Add the eggs and egg yolk, whisk for 2-3 minutes until well combined.

Stir in the flour and espresso powder until a shiny, thick batter forms.

Fold in the chopped chocolate with a rubber spatula.

Pour the batter into the prepared baking pan and bake for 22-30 minutes until the top is crinkly and a toothpick comes out with wet crumbs.

Let the brownies cool completely in the pan before removing and slicing.