Place chopped chocolate in a medium heat proof bowl. Set aside.

Place the butter in a small saucepan over medium heat, stirring occasionally until melted. Continue cooking until it starts to crackle and foam. 

Once it starts to crackle and foam up, stir frequently until the butter emits a nutty aroma and is deep amber in color. Immediately pour over the bowl of chopped chocolate, being sure to include the brown bits at the bottom. Stir until smooth and set aside to cool slightly.

In a separate bowl, sift together the flour, cocoa powder, espresso powder, and salt. Set aside.

In the bowl of a stand mixer fitted with a whisk attachment, add the eggs and vanilla extract. Whisk on medium speed until combined.

On low speed, gradually pour in both sugars. Increase to medium-high speed and whisk for 2 minutes until thick and fluffy.

Once the eggs have whisked, lower the speed to the lowest setting and add the chocolate/butter mixture. Mix until lightly combined (there should be a few streaks).

Add the flour mixture and mix until just combined. Remove the bowl from the stand mixer and scrape the bottom and sides with a rubber spatula to ensure everything is mixed in. Be careful not to over mix in order to prevent deflating the eggs.

Transfer batter to the prepared baking pan and bake for 30-35 minutes (ours were perfect at 32.5 minutes), rotating halfway through. Bake until a toothpick inserted into the center comes out with a few fudgy crumbs.

Cool completely in the pan before removing and cutting into squares.