Plus: More Dessert Recipes and Tips
Preheat the oven to 350°. Line 18 muffin cups with paper liners. In a large bowl, sift the cake flour with the sugar, cocoa, baking soda and salt. Using an electric mixer at low speed, beat in the eggs, melted butter, buttermilk and vanilla. Gradually beat in the hot water until the batter is smooth.
Spoon the batter into the muffin cups, filling them three-quarters full. Bake for 18 minutes, or until the cupcakes are springy to the touch and a toothpick inserted in the center of one comes out with just a few moist crumbs attached. Let the cupcakes cool in the pan for 10 minutes, then transfer them to a wire rack to cool completely.
In a large bowl, cream the butter with the confectioners' sugar at medium speed. Add the melted chocolate and the cream and beat until smooth and fluffy, about 2 minutes. Transfer the frosting to a pastry bag fitted with a large plain or star tip. Pipe the frosting onto the cupcakes. Press 3 marshmallows and a few walnut pieces into the frosting on each cupcake.
Place the chopped chocolate in a small, deep microwave-safe bowl and microwave at high power in 30 second intervals until melted. Whisk in the vegetable oil and let cool slightly. Dip the top of each cupcake into the melted chocolate to completely coat the frosting; let any excess chocolate drip back into the bowl. Transfer the cupcakes to a baking sheet and refrigerate briefly to set the chocolate, about 20 minutes.
The cupcakes can be refrigerated in an airtight container for up to 3 days.