Preheat the oven to 350°F. Line a 9X13 baking dish with aluminum foil, overhanging the edges 2 inches. Spray with nonstick cooking spray.
Whisk together flour, salt, and cocoa powder in a small bowl and set aside.
Set a medium, shatterproof bowl over a pan of just simmering water. Add chocolate, butter, and instant espresso. Stir occasionally until chocolate and butter are melted and the mixture is smooth. Turn off the heat under for the simmering water, still keeping the bowl over the water and add both sugars and mix until well combined. Remove bowl from the top of simmering water.
Add 3 eggs to the chocolate mixture and mix until just combined. Add the other 2 eggs and vanilla and mix until just combined. Do not overbeat the batter at this stage or your brownies will be cakey.
Add the flour mixture to the chocolate mixture and fold it in with a rubber spatula (Do Not use a whisk) just until the mixture is mostly combined with a bit of the flour mixture is visible.
Spread batter evenly into the prepared baking dish and bake for 30 minutes or until a toothpick inserted into the center of brownies comes out with just a few moist crumbs.
When the brownies are just out of the oven make the frosting immediately. Combine the milk, butter, and sugar together in a saucepan and bring to a boil; allow the mixture to cook at a rolling boil for 30 seconds then remove from heat.
Add the chocolate chips and vanilla to the milk mixture and stir until the chips are completely melted. Pour immediately over the just baked, warm brownies. Allow frosted brownies to cool completely in baking pan before cutting.