Preparation Steps

• Make the frosting base first: In a stand mixer bowl, beat the butter on medium-high until very pale and fluffy, 3 to 4 minutes.
• In a separate bowl whisk together the cocoa powder, brownie mix, 2 cups of the powdered sugar, and a pinch of salt. With the mixer on low, add this mixture to the butter. Stream in 2 tablespoons of the heavy cream to help the cocoa bloom; mix until smooth.
• Add the remaining powdered sugar gradually, alternating with the last 2 tablespoons cream and the vanilla. Beat on high 2 minutes, then mix in the honey until glossy. Cover and refrigerate 10 to 15 minutes to set while you bake the cupcakes.
• Position a rack in the center of the oven and heat to 350°F. Line two standard 12-cup muffin tins with 24 paper liners.
• Whisk the milk, sour cream, oil, eggs, and vanilla in a large bowl until smooth. Stir in the espresso powder and salt until dissolved.
• Add the cake mix to the bowl and fold with a spatula just until combined, 30 to 45 seconds. Let the batter rest for 5 minutes to hydrate for a more tender crumb.
• Scoop the batter into the liners, filling each about 2/3 full. Firmly tap the pans once on the counter to release air bubbles.
• Bake 16 to 18 minutes, rotating pans halfway through, until the tops spring back when lightly touched and a toothpick shows a few moist crumbs.
• Cool in the pans for 5 minutes, then transfer cupcakes to a wire rack and cool completely, about 30 minutes.
• Rewhip the chilled frosting for 20 to 30 seconds. If needed, adjust with a teaspoon of cream for a smooth, pipeable texture. Pipe tall swirls onto the cooled cupcakes with a large open-star or round tip.
• Warm the hot fudge until pourable. Drizzle over the frosting and nestle 2 to 3 small brownie chunks on each cupcake. Serve at room temperature.