Preheat oven to 350° F. Lightly grease a 3" x 3" aluminum pan.
Place butter and bittersweet chocolate in a small microwave safe bowl. Heat in the microwave for 30 seconds or until completely melted (after the first 30 seconds, use 10 second intervals to avoid scorching the chocolate)
In a separate small bowl, whisk together the brown sugar, granulated sugar, espresso powder, vanilla, egg and salt until very well combined (the mixture should be thick, light and most of the sugar should have melted).
Using a small spatula, fold in the flour, cocoa powder and any additions such as nuts or chocolate chips Do not over mix.
Pour batter into your prepared pan and smooth with a spatula. Place in the preheated oven and bake for about 15 minutes. The edges will be set, the top will be dry, but the center may sill look soft.
Allow to cool for about 5 minutes before removing from the pan.