Melt the white chocolate bark (use the package instructions).
Scoop about 2 tablespoons of the tempered chocolate into each of the silicone molds. You may need more if you used a mold with really large half-spheres.
Make sure that the entire half-sphere is well covered with a thick layer of chocolate. If your chocolate goes on thick you can just do one coat. If you feel it is too thin, and you think it may break apart easily, you can do two coats, placing the mold in the fridge for about 10 minutes after the first coat.
Transfer the mold to the refrigerator for about 10 minutes.
Once the bark has hardened completely, remove the chocolate from the mold. I recommend using food-safe disposable gloves if you don't want to leave behind any fingerprints. If your rims are a little uneven, heat a small non-stick pan over low heat. Melt the rims just so slightly in order to even out the rims. You'll just need a second or two.
Fill one cup with 2 tablespoons of the hot chocolate powder.
If you haven't already - over medium-low heat, heat a small non-stick pan. Place the rim of one unfilled chocolate dome on the pan for just 2-3 seconds, or until you can see it start to melt a bit.
Place the two halves together and press gently to seal them together. Drizzle with chocolate and decorate with sprinkles, if desired.
Repeat with the remaining half spheres.