Preheat your oven to 350°F (175°C) and line a muffin pan with 12 cupcake liners. In a large mixing bowl, whisk together the flour, granulated sugar, cocoa powder, espresso powder, baking powder, and baking soda until well combined. In another bowl, mix the buttermilk, oil, eggs, and vanilla extract until smooth. Gently fold the wet ingredients into the dry, stirring until just combined. Pour the batter evenly into the liners and bake for 16-18 minutes, or until a toothpick inserted comes out clean. Allow them to cool completely on a wire rack.

While the cupcakes cool, prepare the marshmallow fluff frosting. In a saucepan, combine granulated sugar, light corn syrup, and water; bring this mixture to a boil over medium heat. As the syrup cooks, whip your egg whites in a mixing bowl to soft peaks, then add cream of tartar. Once the sugar mixture reaches 240°F, gradually pour it into the whipped egg whites while beating continuously until you achieve a thick, glossy frosting. Pipe this fluff generously onto the cooled cupcakes, creating a delightful height. Chill them in the refrigerator for 30 minutes for the frosting to set.

Once the marshmallow frosting has set, it's time to coat the Hi Hat Cupcakes. In a microwave-safe bowl, melt your chocolate chips in intervals, stirring between each until completely smooth. Add the coconut oil to the melted chocolate and mix until well combined and thinned to a dipping consistency. Carefully dip each frosted cupcake into the chocolate mixture, allowing the excess to drip back into the bowl. Place the coated cupcakes on a wire rack to let the chocolate set, creating a beautiful, glossy finish.

After the chocolate coating has hardened, your Hi Hat Cupcakes are ready to be served! Present them on a lovely platter.