Butter the sides and bottom of a 9x13-inch pan. Line the bottom with a strip of parchment paper for easy removal.
In a medium bowl, whisk together the flour, the salt, and cocoa powder.
Configure a large sized double boiler.
Place the chocolate, the butter, and the instant espresso powder in the bowl of the double boiler and stir occasionally until the chocolate and butter are completely melted and combined. Turn off the heat, but keep the bowl over the water of the double boiler and add both sugars.
Whisk the sugars until completely combined and remove the bowl from the pan.
Mixture should be room temperature.
Add three eggs to the chocolate/butter mixture and whisk until just combined.
Add the remaining eggs and whisk until just combined.
Add the champagne and orange juice concentrate and stir until combined. Do not overbeat the batter at this stage or your brownies will be cakey.
Sprinkle the flour/cocoa/salt mix over the chocolate. Using a spatula, gently fold the dry into the wet until there is just a trace amount of the flour/cocoa mix visible.
Pour the mixture into the pan and smooth the top with your spatula.
Bake the brownies for 30 minutes (rotate the pan half-way through baking) and check to make sure the brownies are completely done by sticking a toothpick into the center of the pan. The brownies are done when the toothpick comes out with a few moist crumbs.
Cool the brownies completely.
Meanwhile, prepare your champagne glaze.
Mix together the 1/2 cup of champagne and the tablespoon of orange juice concentrate; add the confectioners' sugar a little at a time until it has reached your desired consistency (you might use more or less than 1 cup).
Spoon a bit of the glaze on top of each brownie directly before serving.
