Preheat the oven to 325 degrees F, mist a 9x9-inch or 9x13-inch pan with non-stick spray, and line it with a sheet of parchment.
Place the sugar, cocoa, and butter in a large microwave-safe bowl, and microwave in one-minute increments, stirring, until the butter is completely melted and the mixture feels quite hot to the touch (approximately 3 to 4 minutes).
Add the eggs, one at a time, stirring vigorously after each addition.
Fold in the flour, baking powder, espresso powder, and salt until just a few streaks remain.
Stir in about ⅔ of the chocolate chips and/or walnuts, and vanilla, and transfer the batter to the prepared pan.
Sprinkle the remaining chocolate chips on top, and bake until set around the edges but still soft towards the middle (approximately 25 to 35 minutes).
Cool completely in the pan, then transfer to a cutting board and cut into squares.
** FROSTING
In a medium saucepan over medium-low heat, melt the butter.
Add cocoa powder, milk, and espresso powder.
Stir until mixture becomes thick and is JUST starting to boil
Remove from heat and add powdered sugar
Pour over brownies, spreading quickly before it sets.
