Preheat the oven to 350 degrees.
In a mixing bowl beat butter, sugar, vanilla until fluffy. About 3 minutes, scrape down the sides of the bowl.
Add eggs one at a time and beat until incorporated.
In another bowl add flour, cocoa powder, hot chocolate mix, salt, baking powder and baking soda.
Measure milk and oil together in a measuring cup.
Alternate adding in the flour mix and the milk and oil mixture to the butter and beat on low.
Scrape down the sides and bottom of the bowl and mix until smooth.
Line a cupcake pan and fill each cup 3/4 full with batter.
Bake for 15 minutes or until the cupcakes are not jiggly in the center. Do not over bake.
Let the cupcakes cool, then remove a small hole in the center using a little spoon.
Add marshmallow fluff into a piping bag (or a ziplock and cut the corner) and fill the center of the cupcakes. You can always make the marshmallow fluff the day before or use store-bought.
Using a cookie scooper, scoop a ball of frosting over the top of the cupcake.
Add a large marshmallow ontop and dust with hot chocolate powder.