Mix all of the ingredients for the hot cocoa mix together in a bowl and set aside.
Put the chocolate chips into a microwave-safe bowl and heat in the microwave for no more than 30 seconds at a time, stirring thoroughly (at least 1 min) in between, until the chocolate is fully melted.NOTE: Don't burn the chocolate! This shouldn't take more than 2 turns in the microwave. Most of the melting will actually happen in between heating, when you stir the chips in the warm bowl. Apply ~1 tbsp of melted chocolate to each well and use a pastry brush to spread the chocolate around, coating the walls of each half-sphere evenly.
Refrigerate the molds for 5 minutes until they are just set.
Remove the molds from the refrigerator. Using the leftover chocolate from the bowl, add a small layer around the top rim of each half-sphere, just inside the well, to reinforce the chocolate where the cocoa bombs will be sealed.
Refrigerate the molds for 5-10 minutes.
Prepare any add-ins you have selected. Make sure bags and containers are open and easily accessible, so you can move quickly in the next steps.
Heat your griddle or a wide, flat frying pan until the surface is just hot to the touch, sufficient to melt chocolate.
Remove the molds from the refrigerator. Work carefully to pop each half-sphere of chocolate free of the silicone molds.
Take a half-sphere and press the edges down lightly on the hot griddle or pan, holding for a few seconds until the chocolate has just begun to melt.
Working quickly, fill the shell with 1 tbsp of the hot cocoa mix, plus ~6 mini marshmallows and any other add-ins you prefer
Take a second half-sphere and heat the edges as before, then press it tightly to the half-sphere you just filled, to form a complete chocolate sphere. Hold for a few seconds to let the chocolate cool and harden. Using your finger or a rubber spatula, smooth the melted chocolate along the seam.
Repeat the steps above with the remainder of the half-spheres, until you have 6 complete hot cocoa bombs! If desired, drizzle with more melted chocolate and add sprinkles for decorations.
To enjoy: Simply place the hot cocoa bomb in a mug and pour hot milk over top. Watch as the hot milk dissolves the chocolate, and your marshmallows and cocoa ingredients spring free!