Marshmallow Creme Filling:
- 6 Tbsp. unsalted butter (room temperature)
- 1-1/2 cups confectioner's sugar
- 1/2 tsp. vanilla extract
- 1/2 tsp. salt
- 3/4 cup Marshmallow Fluff
- 2 Tbsp. heavy cream
Make batter: Heat oven to 325 degrees F. Spray 12-cup muffin tin with non-stick spray. Combine flour, baking soda, and salt in bowl. Whisk water, cocoa, chocolate chips, and coffee in large bowl until smooth. Add sugar, sour cream, oil, eggs, and vanilla and mix until combined. Whisk in flour mixture until incorporated. Divide batter evenly among muffin cups. Bake until toothpick comes out with a few dry crumbs attached, 18 to 22 minutes. Cool cupcakes in tins 10 minutes, then turn out onto wire rack and cool completely.
For the Filling: In a bowl of a stand mixer, beat together the butter, confectioner's sugar, vanilla extract, salt and marshmallow fluff. Add the cream and beat just until smooth. Transfer the filling to a pastry bag fitted with a 1/4-inch round tip. Insert tip into top of cupcake and pipe into center being careful not to overfill.