Make the Donuts:

1. Preheat the oven to 350ºF.
2. In a large bowl, combine the flour, cocoa powder, baking soda, baking powder, and salt.  
3. In a separate medium-sized bowl, whisk together the sugar, milk, eggs, and vanilla extract until well combined. 
4. Add the wet mixture to the dry mixture and mix with a spatula until just combined. 
5. Add the melted butter and fold to combine until no streaks of butter are left. 
6. Place the batter in a piping bag or plastic bag. Pipe into a donut pan until each round is 2/3 full. 
7. Bake for 12-15 minutes or until a toothpick comes out clean. 

For the Glaze and Topping:

1. Pour the instant hot chocolate powder into a small bowl or on a flat plate. Set aside.
2. Mix together the powdered sugar, marshmallow fluff, milk, vanilla extract, and salt in a small bowl until well combined. 
3. Once the donuts are fully cooled, dunk the top each one in the glaze, then lightly press into the hot chocolate powder to coat the glazed portion. 
4. Let set on a cooling rack with parchment underneath to catch any drips.