Make the Brownies:

• Preheat your oven to 350°F (175°C). Grease an 8-inch square baking pan and line it with parchment paper, leaving some overhang for easy removal.
• In a microwave-safe bowl, melt the unsalted butter and chopped semisweet chocolate in 20-second intervals, stirring after each, until smooth. Let the mixture cool slightly.
• In a large mixing bowl, whisk together the granulated sugar and eggs until pale and frothy (about 2 minutes). Stir in the vanilla extract and the melted chocolate mixture.
• Sift the cocoa powder, instant coffee granules, flour, and salt into the wet mixture. Gently fold everything together until just combined. Avoid overmixing to keep the brownies fudgy.
• Pour the batter into the prepared pan and spread it evenly. Bake for 20–25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Allow the brownies to cool completely in the pan before frosting.

Prepare the Marshmallow Frosting:

• In a mixing bowl, beat the softened butter with an electric mixer on medium speed until light and fluffy (about 2 minutes).
• Add the marshmallow fluff, vanilla extract, and a pinch of salt. Beat until fully combined.
• Gradually mix in the powdered sugar, starting on low speed and increasing to medium-high once incorporated. Beat until the frosting is smooth, fluffy, and light (about 2 minutes).

Assemble the Brownies:

• Once the brownies are completely cool, spread the marshmallow frosting evenly over the top using an offset spatula. Create decorative swirls for a professional look.
• Dust the frosted brownies lightly with cocoa powder for a hot chocolate-inspired touch.
• Use the parchment paper overhang to lift the brownies out of the pan. Slice into squares and serve.

Store Leftovers:

• Place brownies in an airtight container at room temperature for up to 3 days, or refrigerate for up to 1 week.