Place two 6-cavity silicone hemisphere molds on a baking sheet. These molds should have cavities about 2 inches in diameter.
Using a food-safe paintbrush, paint a thin layer of melted chocolate on the inside of each half-sphere of the molds. When first layer of chocolate is almost set, paint a second layer. Be sure to build up a lip where half-spheres will meet since they may crack if too thin when removed from molds.
Refrigerate chocolate-painted silicone molds for five minutes.
Carefully remove chocolate half-spheres from silicone molds. Wear gloves while doing this to avoid fingerprints on the chocolate or melting it with the heat from your hands.
Prepare half of your half-spheres by adding one tablespoon of hot chocolate mix per cavity. Also add mini marshmallows or sprinkles as desired.
Heat a small nonstick pan on low, just until warm. Remove pan from heat (it need only be slightly warm). Alternatively, use an oven-warmed dish or baking sheet.
Carefully touch flat side of one empty half-sphere on the heated pan for just a second so the edge begins to melt. Quickly adhere to one of the filled half-spheres to seal in fillings. Use a gloved finger to wipe away any excess melted chocolate, or use sprinkles to cover seams.
Repeat Step 6 with remaining half-spheres.
Decorate as desired.