In a small saucepan over medium heat, melt the unsweetened baking chocolate and the butter. Allow to cool for 15 minutes.
2 ounces (40g) unsweetened baking chocolate,4 Tablespoons (57g) unsalted butter
In a medium bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, combine cooled chocolate/butter mixture, concentrated coffee, and milk or cream (start with 1 Tablespoon). Turn the mixer onto low and slowly add the powdered sugar. Increase mixer speed to medium and beat until completely combined, scraping down sides as needed. Add salt to taste.
1 Tablespoon (15mL) concentrated coffee,1 to 2 Tablespoons (15-30mL) milk or cream,2 cups (240g) powdered sugar,pinch of salt
When you are ready to frost the brownies, remove the brownies by lifting the foil overhang straight up. Peel back remaining foil and place brownie block onto a large cutting board or safe-to-cut surface.
Spread frosting evenly over the top of the brownies, then cut into individual servings. Store leftover brownies at room temperature up to 1 week. Brownies freeze well, up to 3 months. Thaw in refrigerator overnight.