Instructions

1. Preheat oven to 350 degrees F. Line a 12 cup and a 6 cup muffin pan with 15 paper liners.
2. Whisk the flour with the cocoa powder, baking soda, baking powder, and salt.
3. In a large bowl beat the butter 2 minutes until smooth and creamy. On medium speed gradually add the sugar, beating until light and smooth 5 minutes. On low speed beat in the eggs and yolk one at a time. Beat in the vanilla.
4. On low speed add the flour in 4 additions beating well after each one. Drizzle in 2 tablespoons of the hot water at a time, mixing briefly until a smooth batter forms.
5. Fill the cupcake liners with ¼ cup of batter each. Bake 18-20 minutes until a toothpick comes clean from center. Cool in pans on wire rack 10 minutes then remove and cool completely on wire racks. If your cupcakes rise too high and aren't flat on top you can turn them on their side and slice off the tops with a serrated knife to make them level. Save them and make mini whoopie pies with the leftover filling.
6. Whisk the marshmallow fluff and butter together in a stand mixer fitted with the whisk attachment. Transfer to a pastry bag with a wide tip. Stick the tip down into the center of each cupcake and squeeze out 1-2 tablespoons of filling.
7. Bring the heavy cream to a simmer in small saucepan. Pour over top of the chocolate chips in a heat proof bowl. Allow to sit 2-3 minutes then whisk until smooth. Dollop a big spoonful of ganache over top of each cupcake and spread from edge to edge using the back of the spoon. Allow to set 15-20 minutes.
8. Whisk confectioners' sugar with a teaspoon of milk, add more as needed to make a thick icing. Drizzle over top or use a pastry bag with a small tip to decorate the top. Keep refrigerated.