For the Mocha Brownies:
Preheat the oven to 350°F (175°C). Line an 8×8 inch baking pan with parchment paper, leaving an overhang around the edges for easy removal. Alternatively, line with aluminum foil and lightly grease.
In a large microwave-safe bowl, combine the chopped chocolate, butter, and instant coffee powder. Microwave on medium power in 45-second intervals, stirring between each interval, until fully melted. Alternatively, melt in a double boiler over low heat.
Let the mixture cool slightly, then whisk in the granulated sugar, whisked eggs, and vanilla extract.
Gently fold in the flour and salt until just combined.
Pour the batter into the prepared pan and spread it evenly. Bake in the preheated oven for 23-28 minutes, or until the top is set and a toothpick inserted in the center comes out with only a few moist crumbs.
Let the brownies cool completely in the pan on a wire rack before frosting.
For the Cafe Latte Buttercream:
Dissolve the instant coffee in the boiling water and let it cool completely. You can place it in the fridge to speed up the process.
In a large mixing bowl, beat the softened butter with an electric mixer for about 1 minute until creamy.
Turn the mixer to low speed and gradually add 1 cup of powdered sugar, along with the vanilla extract and salt.
Beat in the cooled coffee mixture.
Continue adding the rest of the powdered sugar, ½ cup at a time, until the desired sweetness and consistency is reached. If the frosting is too thick, add 1 tablespoon of whipping cream.
Once the brownies have cooled, remove them from the pan using the overhanging parchment paper and place them on a cutting board. Frost the brownies with a flat-edge knife.
Slice the brownies with a sharp knife and serve.