Step 1

Set up a double boiler. Fill a 4-quart pot halfway with water and set over medium-high heat. Find a heat proof bowl that fits securely on top of the pot without touching the water.

Step 2

Chop and temper the chocolate. If using bars, finely chop 12 ounces semisweet chocolate. Set 1/3 aside (about 4 ounces). Transfer the remaining 2/3 (8 ounces) to the bowl and fit over the double boiler. Melt the chocolate in the double boiler, stirring regularly with a silicone spatula to help with melting and keep it from getting too hot. Keep an eye on the chocolate temperature — it should never get too hot to touch but you can check this using a digital probe thermometer — do not exceed 100° F. Remove the bowl from the double boiler. Add the reserved chocolate in 3 increments, stirring until incorporated and melted after each addition. Continue mixing until the chocolate is between 88 and 91°F. You can check this with a digital probe thermometer or by touch — the chocolate should be about body temperature, so test on the back of your hand or wrist.

Step 3

Coat the molds with the tempered chocolate. Drop 1 tablespoon tempered chocolate into each well of 2 (6-count, 2-inch wide) half sphere silicone mold trays. Use a brush to spread the chocolate up the sides and over the edges of the molds by just a bit. Let sit at room temperature until the chocolate is hardened, 5 to 15 minutes. Brush the wells with more tempered chocolate (about 1 teaspoon each) to achieve a thick, even coating. Let sit at room temperature until completely hardened, 15 to 30 minutes.

Step 4

Fill half the spheres with cocoa and marshmallows. Place 1 tablespoon hot cocoa mix, 1 heaping tablespoon mini marshmallows, and 1/2 teaspoon sprinkles if desired into each of half of the wells (6).

Step 5

Paint or pipe a border of chocolate on the unfilled shells and assemble the bombs. Wearing nitrile or latex gloves, gently release the remaining 6 unfilled shells from their molds, but keep them resting in the well. Use a pastry brush or a small piping bag to coat their edges with tempered chocolate. (If the chocolate has cooled and lost its temper, place it back briefly over the double boiler until warmed back to between 88 and 91°F.) Top each marshmallow-filled shell with a coated, unfilled shell to form a sphere. Let sit at room temperature to harden and set, at least 15 minutes.

Step 6

Coat cupcake liners with chocolate. Drop 1 tablespoon tempered chocolate into each of 6 parchment paper cupcake liners. Use a brush to coat the bottom and sides of the liners with the chocolate. Let sit at room temperature until the chocolate is hardenened, 5 to 15 minutes. Brush the liners with more tempered chocolate (about 1 teaspoon each) to achieve a thick, even coating. Let sit at room temperature until the chocolate is completely hardened, 15 to 30 minutes.

Step 7

Fill with cocoa and marshmallows. Place 1 tablespoon hot cocoa mix, 1 heaping tablespoon mini marshmallows, and 1/2 teaspoon sprinkles if desired into each liner.

Step 8

Close the bombs with more chocolate. Drizzle 1 tablespoon tempered chocolate over the filling in each liner. Use the back of a small spoon to smooth the chocolate over the filling to completely enclose it. Let sit at room temperature until the chocolate is completely hardened, 15 to 30 minutes. (If the chocolate has cooled and lost its temper, place it back briefly over the double boiler until warmed back to between 88 and 91°F.) Wearing nitrile or latex gloves, gently release the chocolate bombs from the liners.

Step 9

Drizzle with any remaining chocolate and add sprinkles (optional). Before storing, you can decoratively drizzle the bombs with any remaining chocolate and add sprinkles to garnish if desired. Use gloves when decorating or moving the cocoa bombs to prevent fingerprints.

Step 10

Serve the chocolate bombs. Heat 1 cup milk for each hot chocolate bomb gently on the stovetop or in the microwave for about 2 minutes until very hot, almost to a boil. Pour the hot milk into a large mug and gently add the bomb for the most dramatic explosion.