01 -
Preheat oven to 350°F (175°C). Line an 8-inch square metal baking pan with parchment paper.
02 -
In a small saucepan over medium-low heat, melt the butter. Swirl the pan occasionally and increase the heat to medium. Continue cooking until the butter becomes foamy and emits crackling sounds. When the crackling stops, swirl the pan until the butter develops a nutty aroma and amber brown bits form at the bottom, about 2 to 3 minutes. Remove from heat and pour into a mixing bowl, including the brown bits.
03 -
Immediately add the chopped semisweet chocolate to the warm butter and stir until it is fully melted.
04 -
In the bowl of an electric mixer fitted with the whisk attachment, combine granulated sugar, brown sugar, eggs, and vanilla extract. Beat on high speed until thickened, fluffy in texture, and lightened in color, about 8 minutes.
05 -
Reduce mixer speed to low and gradually pour in the warm brown butter and chocolate mixture. Mix until combined.
06 -
Use a rubber spatula to fold in the flour, cocoa powder, espresso powder (if using), and salt until just combined. Stir in the chocolate chips.
07 -
Pour the batter into the prepared pan and smooth it out with a spatula. Bake for 22 to 25 minutes, or until set but not overbaked. Allow brownies to cool completely in the pan before slicing and serving.