Preheat oven to 350ºF. Prepare a 13 x 9" baking pan by lightly spraying with a non-stick cooking spray and lining with parchment paper or aluminum foil.
In a medium saucepan over medium heat, melt butter and chopped chocolate, stirring constantly to prevent scorching. Once melted, remove saucepan from heat and stir in cocoa powder, sugar, salt, vanilla, and espresso powder (if using) with a wooden spoon. Slowly pour in eggs, but stir quickly to avoid scrambling them. Fold in flour only until incorporated. Batter will be thick.
Pour into prepared baking pan and smooth over using an offset spatula. Bake for 25 minutes. Remove from oven and cool completely before cutting into squares. Once completely cooled and cut, store in an airtight container.