Preheat the oven to 350°F and line two standard muffin tins with cupcake liners.
In a stand mixer, cream the butter, sugar, and vanilla together until smooth.
Add the egg whites one at a time, mixing well after each addition. Scrape the bowl as needed.
Mix in the flour, hot cocoa powder, and baking powder until just combined. Gradually add the milk and mix until the batter is smooth.
Divide the batter evenly between the cupcake liners, filling each about halfway.
Bake for 14-16 minutes, or until the edges are slightly golden and a toothpick inserted in the center comes out clean. Let cupcakes cool completely.
To make the frosting, cream the butter, salt, vanilla, and half the powdered sugar on low speed in a stand mixer until combined. Increase speed and mix until smooth.
Add the marshmallow cream and mix until fully incorporated. Scrape the sides of the bowl as needed.
Mix in the remaining powdered sugar and milk, starting on low speed and increasing to whip the frosting until light and fluffy.
Transfer frosting to a piping bag fitted with a large star tip. Pipe swirls onto the cooled cupcakes.
Garnish with mini marshmallows and chocolate chips, if desired.