Sift together the flour, hot chocolate mix, baking powder, and salt and set aside.
Cream the butter and sugar using a hand mixer or stand mixer, until it's light and fluffy.
Add eggs one at a time, mixing well after each addition. (Tip: I always crack my eggs into a small bowl or cup, just in case some shell drops. It's much easier to fish shell out of a cup with just egg than it is to fish it out of a cake mixture.)
Add vanilla and mix until incorporated.
Add the flour mixture ¼ cup at a time, adding a little bit of milk after each addition of flour. Alternate adding the flour and mixture until all of both are incorporated. Continue to mix on a low speed until everything is well-incorporated.
Scoop the batter into the cupcake pan with each cup being about ⅔ of the way full.
Bake for about 20-25 minutes until a toothpick comes out clean.
Make the frosting by combining the chocolate frosting and marshmallow fluff.
Allow the cupcakes to cool completely before frosting.
Use a piping bag with the tip of your choice to frost the cupcakes (I used Wilton #32).