Hot Chocolate Bombs

Melt 4 ounces (about 2/3 cup) semisweet chocolate on the stovetop in a double boiler or in the microwave in 30-second intervals, stirring between each interval.

Pour 1 teaspoon melted chocolate into each of 16 (1-ounce) cups in a silicone hemisphere mold. Use the back of a small spoon, such as a 1/4-teaspoon measuring spoon, to push the melted chocolate up the sides and around the edges of each cup to cover completely. Place the mold on a rimmed baking sheet and freeze until the chocolate is completely hardened, about 5 minutes.

Melt the remaining 2 ounces (1/3 cup) semisweet chocolate. Working with 1 cup at a time, pour in 1/2 teaspoon melted chocolate. Spread the chocolate on the sides and edges, coating them well to prevent cracking. Repeat with the other cups. Freeze the mold for another 5 minutes.

Whisk cocoa and sugar together in a small bowl. Line a rimmed baking sheet with parchment paper and top with an upside-down wire rack, which will keep the bombs from rolling while you're filling them. Remove the chocolate shells from the mold by gently peeling the silicone away from the edges of the chocolate with your thumbs and pushing from the bottom of the mold with your index fingers.

Heat a microwave-safe plate in the microwave just until warm, about 30 seconds. Place 1 chocolate shell, open-side down, on the warm plate, and melt the edges until flat. Carefully spoon 1/2 teaspoon cocoa-sugar mixture into the shell. Place the filled shell on the rack to hold it upright, then repeat the melting process with a second shell. Fill that half with marshmallow bits (about 3/4 teaspoon). Carefully press the two halves together. (Don't worry; the contents will stay in place.) Use your finger to spread the melted chocolate around the seam of the bomb to seal it closed. Repeat until all the bombs are filled and sealed.

Melt white chocolate and place in a small zip-top sandwich bag. Snip a small corner off the bag, then drizzle the chocolate over the bombs. (Refrigerate the hot chocolate bombs in an airtight container for up to 1 month. Bring to room temperature before serving.)

To serve, heat milk in a saucepan over medium-low heat just until steaming hot. Place a hot chocolate bomb in a mug and pour 3/4 cup steaming milk over the bomb. Stir well until the chocolate is melted and fully dissolved. Serve with additional marshmallow bits, if desired.