
Fill a 4-quart pot with water and set over medium-high heat. Place a heat proof bowl sitting securely on top of the pot without touching the water.
Place the chopped chocolate in the bowl and stir with a silicone spatula until the chocolate melts, about 5 minutes. Remove the chocolate immediately after it melts, being careful not to overheat it, and let it cool for 5 minutes.
Place 1 tablespoon of the tempered chocolate into each of the silicone mold cups (12 total). Brush a thin layer of the chocolate up the sides and slightly over the edges of the molds by just a bit. Let it sit at room temperature until the chocolate hardens, 5–15 minutes. Apply another thin coating of the tempered chocolate and let it sit at room temperature again until the chocolate hardens, about 15–30 more minutes.
Carefully remove all the chocolate from the molds. Divide the cocoa mix and marshmallows into 6 of the molds, filling all the way to the top.
Use a Q-tip to outline a border of the melted chocolate on the 6 unfilled chocolate molds. Place an unfilled mold on top of a filled mold and close to form a sphere. Repeat for the remaining 5 hot cocoa bombs and let sit at room temperature again until the chocolate hardens and sets, about 30 more minutes.
When ready to use, serve with warm milk.