Place white chocolate in a double boiler over simmering water. Melt, stirring occasionally until smooth. Keep warm while you work.
If coloring, remove just enough melted white chocolate to a small mixing bowl one at a time, depending on how many colors you're making. Add a few drops of food coloring to melted chocolate repeating until the desired color is reached. Work quickly, mix well. (You can skip the food coloring leaving it white and decorate with colorful sprinkles, if preferred)
Spoon 2-3 Tbsp melted chocolate into each mold and “paint” the entire mold evenly using the back of a small spoon or a pastry brush. Make sure you get the chocolate all the way to the top of the mold. Once molds are filled, while still wet, use a pastry bench scraper OR the dull side of a knife to scrape across the mold to flatten and smooth the edges.
Allow molds to rest until chocolate is hardened 5-15 minutes. (Tip: Speed up this process by placing molds in the freezer for several minutes). Wearing latex gloves, gently pop chocolate spheres out of molds, being careful not to chip or break them.
Fill one half with 3 generous Tbsp hot chocolate mix and a few pastel marshmallows plus sprinkles, if desired.
Use additional melted chocolate to brush or pipe along the edge to form the edible "glue." Working quickly, press together to seal the two halves and smooth.
Drizzle tops with melted white chocolate and decorate with pastel sprinkles as desired. Let dry completely in a muffin pan.
To serve, place one bomb in an 8 ounce mug and pour hot milk or water over it stirring until melted.
Hot cocoa bombs can be stored in an airtight container at room temperature for up to 3 weeks.