Preheat oven to 350. Line 9×13 dish with parchment, allow overhang to remove.
In a large bowl, add cocoa powder, chocolate, and espresso powder.
In a saucepan over medium low heat, begin to melt butter. Once the solids show, start to watch the pot. Swirl the pot occasionally. You want the solids to brown. When the butter stops foaming and begins to smell nutty, and the solids are a nice caramel brown, remove from heat.
Add browned butter to chocolate bowl, be sure to scrape the browned solids as well. Whisk until shiny and smooth.
With a stand or hand mixer, beat sugars, vanilla, and eggs on medium-high until thick and light in color. This takes anywhere from 8-10 minutes.
Reduce speed to low. Slowly stream in chocolate while the mixer is going just until incorporated.
Using a large rubber spatula, gently fold in flour and salt. Mix until no more dry spots.
Pour batter into prepared pan, spread evenly. Bake for 25 minutes until top is glossy and center is barely set. Center should not be giggly. Remove from oven, let cool in pan for at least 30 minutes before moving them to cut.