Combine the hot water and coffee granules. Combine all of the ingredients, including the coffee, in a large bowl. The batter will be very thick. Spread the batter into a greased 9x13 baking pan. Bake at 350 degrees for 20 minutes. We like to undercook our bars just a bit so they are gooey. If you'd like to, you can bake them another 5 minutes little longer to have a firmer bar.
For the buttercream, cream the butter in a stand mixer for about 3 minutes. Add the powdered sugar, hot cocoa mix, salt, vanilla and heavy cream. Beat on medium speed for 3 minutes, or until fluffy.
Sprinkle the marshmallow bits and mini chocolate chips over the top of the buttercream. Gently press down on top of the marshmallows and chips to press them into the buttercream.
The calories shown are based on the recipe being cut into 20 pieces and all of the frosting used, with 1 serving being 1 frosted brownie. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.