Homemade Hostess Cupcakes (but better)

Whisk oil and sugars until smooth. Add eggs, vanilla, sour cream, and milk. Stir in flour, cocoa powder, baking soda, baking powder, and salt until just combined. Fill liners ¾ full and bake at 350°F for 16–18 minutes. Let cool completely. Beat butter until smooth. Add powdered sugar, vanilla, and salt. Mix in marshmallow fluff, then add heavy cream until light, fluffy, and pipeable. Cut a hole in each cupcake and pipe the filling inside. Don’t hold back. Heat cream until hot, pour over chocolate chips, let sit, then stir smooth. Add vanilla. Dip or spoon over cupcakes and let set. Pipe the classic swirl on top.