3. Reheat chocolate if needed, then add another spoonful to each chilled mold. Let it set in the fridge or freezer. Once set, gently remove the chocolate half rounds from the molds; they should come out easily.
4. Fill each bottom half round with hot cocoa mix and mini marshmallows. For melting chocolate, warm a plate on the stovetop over boiling water or in the microwave until it's sufficiently hot for melting.
5. Work with one bomb at a time. Press a top half round against a heated plate to melt the rim slightly. Seal it over the filled bottom half round. Repeat for the other two bombs. If seams are loose or messy, warm a spoon (dip it in hot water or run it under a hot faucet) and run the back along the seam to melt and seal the edge.