Prepare a double boiler by filling a 4-quart pot halfway with water and heating it over medium-high heat.
Chop and temper the chocolate: finely chop 12 ounces semi-sweet chocolate, set aside 1/3 (4 ounces).
Melt 2/3 (8 ounces) chocolate over double boiler, stirring regularly, keep temperature below 100°F.
Remove from heat and add reserved chocolate in three portions, stirring to melt and cool to 88-91°F.
Option 1: Using silicone molds: Spoon 1 tbsp tempered chocolate into 12 half-sphere wells, brush sides, let harden 5-15 mins.
Brush wells with extra chocolate for thick coating; let harden 15-30 mins.
Fill 6 wells with 2 tsp hot cocoa mix, 3-4 mini marshmallows, 1/2 tsp sprinkles each.
Coat rims of remaining 6 shells, assemble halves, let set 30-45 mins.
Option 2: Without molds: Spoon 1 tbsp tempered chocolate into 6 parchment liners, brush sides, harden 5-15 mins.
Brush liners with more chocolate, harden 15-30 mins.
Fill with 2 tsp cocoa mix, 3-4 mini marshmallows, 1/2 tsp sprinkles.
Drizzle 1 tbsp chocolate over filling, smooth, harden 30-45 mins.
Remove from liners carefully.
Decorate bombs with remaining chocolate and sprinkles if desired.
To serve, heat 1 cup milk until very hot, pour into mug, add one chocolate bomb, stir vigorously as it melts.