Preheat your oven to 350°F (175°C). Grease and line an 8x8-inch baking pan with parchment paper.

In a microwave-safe bowl, melt the unsalted butter and chopped semisweet chocolate together until smooth.

Allow the melted mixture to cool slightly, then whisk in the granulated sugar, eggs, and vanilla extract until combined.

In a separate bowl, whisk together the cocoa powder, instant coffee, all-purpose flour, and salt. Gradually fold these dry ingredients into the wet mixture until just combined.

Pour the batter into the prepared baking pan and smooth the top. Bake for 20-25 minutes or until a toothpick comes out with a few moist crumbs.

Let the brownies cool completely in the pan.

In a mixing bowl, beat together the marshmallow fluff, softened butter, vanilla extract, and a pinch of salt until smooth.

Gradually add powdered sugar while continuing to beat until light and fluffy.

Spread the marshmallow frosting evenly over the cooled brownies.

Dust lightly with cocoa powder, slice into squares, and serve.