For the brownies: Preheat the oven to 375 degrees F. Line an 8-by-8-inch square baking pan with parchment paper so that the paper overhangs two sides of the pan. Coat the parchment and sides of the pan with cooking spray.
Combine the butter and chocolate in the top of a double boiler and melt over medium heat. Set aside to cool slightly. Whisk the eggs in a large bowl until pale yellow. Add the granulated sugar, espresso powder and vanilla extract to the eggs and whisk until fluffy. Whisk together the flour and salt in a small bowl.
Pour the chocolate mixture over the egg mixture and whisk until smooth. Fold in the flour mixture until just combined. Stir in the chocolate chips. Transfer the batter to the prepared pan, spreading it to the edges with an offset spatula or the back of a spoon. Bake until the batter is set and cracked on top, 30 minutes. Remove to a rack and let cool.
For the icing: Whisk together the buttermilk, coffee, confectioners' sugar and cocoa powder until smooth.
Remove the cooled brownies from the pan by using the parchment overhang to lift them out. Pour the icing over the top and spread with a small offset spatula. Let dry. Cut into squares and serve.