Pour the dark chips in a microwave safe bowl. Microwave in 30-second intervals, stirring between each, until chocolate is fully melted (it generally takes about 1:30-2:00 total, depending on your microwave).
Pour a little chocolate into each cavity of a six-count silicone dome shaped mold. Use a spoon to spread the chocolate around the dome so that it thoroughly coats the entire cavity.
Place the mold on a level spot in the freezer for about 10 minutes, or until chocolate is fully solid. Carefully pop out each of the six domes and place on a plate.
To each of three domes, add 1 tablespoon hot cocoa and 1 tablespoon mini marshmallows.
Place a skillet over medium heat for a minute on stovetop, just to get it warm (but not excessively hot). Take one of the empty domes and hold it on the skillet for 1-2 seconds, just enough time to melt the chocolate on edge slightly, then quickly place that on top of one of the filled domes. This should quickly attach and harden, creating a solid circle. Repeat with the remaining two domes.
To serve, heat 1 1/4 cups milk in the microwave in a microwave-safe mug or cup until hot (it should take around 1:30 to 2 minutes, depending on your microwave). Either drop one hot chocolate bomb into this, or place one hot chocolate bomb in a mug and pour the hot milk on top.