In a microwave-safe bowl, melt the milk chocolate chips in 30-second intervals, stirring in between until smooth. Be careful not to overheat.
Using a silicone sphere mold, pour a tablespoon of the melted milk chocolate into each half of the mold. Use a spoon to spread the chocolate evenly, covering the inner sides. Repeat for all molds, then place in the refrigerator for about 10-15 minutes to set.
Once the first layer is solid, repeat the melting and setting process with another layer of milk chocolate to ensure a sturdy shell. Refrigerate again for another 10-15 minutes.
Carefully pop the chocolate shells out of the molds. If some edges are rough, you can gently heat a plate and smooth the edges of the shells by running them along the plate.
Place cocoa powder, a few mini marshmallows, and sprinkles (if using) inside half of the chocolate spheres. Add a small drizzle of melted white chocolate on top of the mixture to help seal the bomb.
Take the remaining half of the chocolate shell and gently press it onto the filled half. Use the melted white chocolate as glue to help them stick together.
Drizzle some melted white chocolate over the top of the assembled hot chocolate bombs and sprinkle with additional sprinkles for a festive look.
To use, place one hot chocolate bomb in a mug and pour hot milk over it. Watch as it melts and releases marshmallows and chocolate! Stir with a spoon to combine.