Sift together the flour, sugar, cocoa powder, baking soda, baking powder, and salt.
¾ cup all-purpose flour, ¾ cup granulated sugar, ⅓ cup cocoa powder, ¾ teaspoon baking soda, ½ teaspoon baking powder, ¼ teaspoon kosher salt
In a separate bowl, mix together the egg, sour cream, vegetable oil, and vanilla with a whisk until combined.
1 large egg, ⅓ cup sour cream, ⅓ cup vegetable oil, ½ teaspoon vanilla extract
Add the wet ingredients to the dry ingredients and mix together with a wooden spoon or rubber spatula. Add the hot coffee or water and carefully stir to combine.
½ cup hot coffee or water
Fill each cupcake liner ⅔ full, and bake for 13-15 minutes. The top of the cupcake will spring back when touched. A toothpick inserted into the center will come out clean with a few moist crumbs.
Cool completely on a wire rack. Once cool, use a cupcake corer to remove the filling or the bottom of a round wooden spoon to push down the filling to make a hole in the center of each cupcake.
Transfer the marshmallow fluff to a piping bag and fill the center of each cupcake with filling to the top.
½ cup marshmallow fluff