Preheat your oven to 350°F (175°C) and prepare an 8-inch baking dish by greasing it or lining it with parchment paper.
Melt the unsalted butter in a microwave-safe bowl until just melted.
In a large mixing bowl, combine the melted butter, granulated sugar, eggs, and vanilla extract until well mixed.
In another bowl, whisk together cocoa powder, flour, salt, and baking powder. Gradually add to the wet ingredients until fully incorporated.
Spread the batter evenly into the prepared pan and bake for 30-35 minutes or until a toothpick inserted comes out clean.
Let the brownies cool completely before preparing the frosting: mix instant coffee granules with heavy cream and set aside; beat unsalted butter until fluffy, then gradually add powdered sugar followed by your coffee mixture until smooth.