Preheat oven to 350º and place the rack in the middle.
Line an 8x8 baking dish with foil.
Combine chocolate and butter in a medium saucepan and cook over medium heat, stirring frequently, until evenly melted.
Remove from heat and let cool to room temperature.
In a large bowl, combine eggs, sugar, cocoa powder, vanilla extract, espresso, and salt and stir until just combined.
Add cooled chocolate and mix until even in color.
Add flour and stir until just combined and no white streaks remain.
Transfer batter to the baking dish.
Bake until a knife stuck in the center comes out clean, 25-30 minutes.
Let brownies cool completely before cutting.