Step 1

Preheat oven to 350 degrees F. Line a metal 8-inch square pan with parchment paper.

Step 2

In a small saucepan set over medium low heat, melt the butter. Swirling the pan occasionally, continue to cook the butter, increasing the heat to medium. It should become foamy with audible cracking and popping noises. Once the crackling stops, continue to swirl the pan until the butter develops a nutty aroma and brown bits start to form at the bottom. Once the bits are amber in color, about 2 to 3 minutes after the popping stops, remove from heat and pour into a mixing bowl, scraping the brown bits into the bowl.

Step 3

Immediately add in the chopped chocolate and stir until melted.

Step 4

In the bowl of an electric mixer fitted with the whisk attachment, combine the granulated sugar, brown sugar, eggs, and vanilla. Beat mixture on high speed until completely thickened and fluffy in texture and lightened in color, about 8 minutes. On low speed, gradually pour in the warm brown butter chocolate mixture.

Step 5

Use a rubber spatula to stir in flour, cocoa powder, espresso powder, and salt until just combined. Stir in the chocolate chips.

Step 6

Pour into prepared pan and smooth out with a spatula. Bake in the preheated oven for 22 to 25 minutes, or until set but not overbaked. Let cool in pan completely before slicing and serving.

Step 7

Brownies can be stored at room temperature for up to 2 days.