Preheat the oven to 350ºF, and spray a mini muffin pan with nonstick cooking spray.

In a large bowl, stir the butter, brown sugar, egg, and vanilla together until combined. Stir in the hot chocolate mix and cocoa powder. Add the flour, salt, and chocolate chips, and stir until just combined.

Divide batter evenly among the mini muffin pan wells, filling each about 2/3 full. Bake in the oven for 10-12 minutes, or until a toothpick inserted in the center comes out clean. Cool for 5 minutes before transferring brownie bites to a wire rack to cool completely.

In a small bowl, stir the hot chocolate mix and water together until smooth and dissolved; cool slightly.

In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and powdered sugar together until creamy. Add the hot chocolate liquid and beat until fluffy, about 2 minutes. Transfer frosting to a pastry bag fitted with a large round tip, or a ziplock bag with the corner cut off.

Pipe frosting onto the brownie bites. Top each brownie bite with a few chocolate chips and mini marshmallows/marshmallow bits. Sprinkle the crushed peppermint candies on top. Serve and enjoy!