Preheat the oven to 350 degrees F.
Melt the butter and chocolate in a double boiler on the stovetop OR in the microwave in 30 second increments, stirring after each time.
When the chocolate and butter are fully melted, beat in the sugar using a wooden spoon or rubber spatula.
Add the eggs one at a time, being sure to fully incorporate the first one before adding the next.
Add the vanilla, espresso powder and salt, then beat for one minute.
Sift in the flour and cocoa powder and mix until well combined.
Pour the batter into a parchment lined 8x8" pan and smooth out the top.
Bake for about 30-40 minutes or until a toothpick inserted in the center comes out clean (a few crumbs are okay, but be sure there is no runny batter).
Allow the brownies to cool at least 30 minutes before cutting. I like to transfer them to the refrigerator until they're ice cold (overnight is best, but this can be sped up in the freezer if desired).