Place chocolate candy melts in a microwave safe bowl and melt in the microwave per the directions on the package.
Place 2 tbsp melted candy melt into one of the gingerbread house molds and, using the bottom of your tablespoon measuring spoon, press the chocolate up the sides of the mold, keeping it thick but even. Repeat with all of the molds.
On a baking sheet, lined with a non-stick silicone baking sheet or parchment paper, draw a small square of chocolate with a decorating bag and small piping tip, large enough to serve as a bottom for the gingerbread houses. Repeat drawing 6 squares.
Working with the first square of chocolate you made, fill it in with a solid filling of chocolate, making it about ¼ inch thick and as flat as you can possibly make it.
Place the chocolate filled molds and baking sheet into the freezer for 5 minutes.
Remove molds from the freezer and gently slide the chocolate out of the mold.
Fill the molds with 2 tbsp hot chocolate mix and 10-15 mini marshmallows.
Press one of the chocolate squares onto the bottom of a house, then using the chocolate in your decorating bag with the small piping tip, form a join line between the chocolate square and the house, joining the two pieces.
Using a sifter, dust the powdered sugar over the assembled hot cocoa bombs or make an icing by adding a teaspoon of milk to your powdered sugar and ice the tops of the houses.
Place the hot cocoa bombs into the freezer for 5 minutes to get them to set, then remove and store in an airtight container until you’re ready to use.