Put 8 ounces of chocolate in a double boiler over low heat to temper it. Heat the chocolate while stirring until it almost melts (about 100 degrees F on an instant-read thermometer). Remove from the fire and whisk until melted before adding the remaining chopped chocolate in two or three batches. A thick, 90°F bar of tempered chocolate will be produced.
Divide the chocolate between two sizable silicone hot chocolate bomb molds with a total of 6 cavities (2 1/2-inch cavities). To evenly cover the sides and the bottom, use a small brush to spread the chocolate up the sides. Make sure the sides are thick enough to reach the mold's edge. Allow it to rest for 20 to 30 minutes at room temperature until thoroughly set. (If the weather is too hot, you can put the molds in the refrigerator for a little while to complete.)
Remove the chocolate half-spheres from the molds with care while wearing latex gloves. Put it on the work surface on a piece of parchment paper. Six of the half-spheres should each have one-half of the hot cocoa, three or four pumpkin marshmallows, one ghost, one spider, and a half to one teaspoon of sprinkles.
Warm a small nonstick skillet over very low heat. To seal the bombs, set an unfilled half-sphere on the skillet open-side down for just a second to warm the chocolate. Set on top of a filled half-sphere, matching the seams as much as possible. Repeat with the remaining half-spheres. Let sit until hardened, about 5 minutes.
Put the regular marshmallows in a small, microwave-safe bowl to create the marshmallow webs. Microwave until the marshmallows inflate and soften, 10 to 15 seconds. With gloved hands, pull up a little bit of marshmallow and stretch between your fingers to make thin webbing. Stretch the webbing over and around the bombs. Continue to pull and stretch the marshmallow until all of the bombs are covered in webbing. Add 2 candy eyes or a spider to each, if desired.
To serve, heat 6 to 8 ounces of milk per bomb until very hot. Add to mugs and drop in the bombs. Enjoy, stirring to melt the chocolate.