Prepare Chocolate Spheres: Spoon melted chocolate into sphere molds, spread around to fill, and let set at room temperature.
Fill Spheres: Fill half the spheres with hot cocoa powder and marshmallows. Use a hot knife to even out the edges of the spheres, then quickly set another half on top to seal.
Serve: Place the chocolate bomb in hot milk and watch as it melts, revealing the cocoa and marshmallows inside.