Line 6 paper cupcake wrappers inside a muffin tin. Sort the unicorn sprinkle mix and set aside 6 horns. Warm a small nonstick skillet over low heat for sealing later.
In a microwave-safe bowl, melt 6 oz of the white chocolate at 50% power in 30-second bursts, stirring between each, until about 75% melted; then stir until fully smooth. If using, whisk in the tiny pinch of sea salt.
Spoon about 1 tbsp melted chocolate into each half-sphere silicone mold (2.5-inch). Tilt and brush to coat the sides evenly. Refrigerate 7–8 minutes, just until set.
Rewarm the remaining chocolate briefly and melt an additional 3 oz. Add a second thin coat, focusing around the rims to reinforce. Chill 8–10 minutes until firm all over.
Gently unmold the shells and let them rest 2 minutes at room temperature to avoid condensation. Lightly touch the rim of one shell to the warm skillet for 1–2 seconds to level the edge, then set it flat-side up in a cupcake wrapper as your base.
Fill each base with about 1 heaping tbsp hot chocolate mix and 4–6 mini marshmallows. Do not overpack; leave headroom for sealing.
Warm the rim of a matching top shell on the skillet for 1 second, cap the filled base, and gently rotate while pressing to seal. Paint a tiny bead of melted white chocolate (from the remaining chocolate) around the seam for extra strength. Chill 5 minutes to set.
Melt 1–2 oz of the reserved white chocolate at 50% power. Dip a unicorn horn into the chocolate and attach it to the top center of each bomb. Cut the white candy melts wafers in half to form small triangles and adhere one on each side for ears. Hold a few seconds until they grip.
For the mane, use a toothpick to paint a small zig-zag or triangle of melted chocolate on the back of the bomb; immediately cover with unicorn sprinkles. Repeat in front if desired. Refrigerate 3–4 minutes to lock everything in place.
Draw closed lashes or tiny eyes on each unicorn using the edible black marker. Let the decoration dry for 5 minutes before moving or packaging.
To serve, set a bomb in a mug and pour 8–10 oz of hot milk over the top. Let it bloom, then stir until smooth. Finish with whipped cream and a few extra sprinkles.