Melt the chocolate chips in the microwave at 30-second intervals until completely smooth.
Place 1 tablespoon of the melted chocolate in the center of each mold. Using a pastry brush, swirl it around the mold to cover all sides. Freeze for 3 minutes.
Remove the mold from the freezer and repeat the process, filling the molds with a tablespoon of melted chocolate at a time and freezing each layer.
Once you've used up the melted chocolate, freeze the shells for 15 minutes.
Carefully remove the spheres from the molds and set aside.
Combine the sugar, cacao powder, and salt in a small bowl.
Divide the mixture and marshmallows between 3 of the chocolate domes.
Warm a plate in the microwave for 30 to 45 seconds.
Place the unfilled sphere halves on the warm plate for 5 seconds to melt their edges.
Sandwich the filled and unfilled spheres together. Decorate the vegan hot chocolate bombs with your choice of toppings.
Refrigerate for at least 5 minutes to set, and you're done. You can also place the hot chocolate bombs in cupcake liners.