Pour candy melts into a heat-proof bowl.
Place 1" of water in a saucepan and bring to a simmer.
Set bowl of candy melts over the simmering water. Note: bottom of the bowl should not be touching the water and the water should not be boiling.
Stir candy melts every few minutes until melted. Remove from heat.
Using a silicone pastry brush, paint the candy melts into the silicone molds. This is a thin coat - a second coat will be added after the first coat sets.
Place molds in refrigerator for 10 minutes or until candy melts are set.
Paint a second coat of candy melts in the molds. Make sure to cover any thin spots. Place molds back in refrigerator for another 10 minutes.
Gently remove the domes from the mold by pushing up gently from the bottom.
Lightly heat the bottom of a cake or sheet pan by placing on a stove burner or using a kitchen torch. Place one of the domes, round side up, on the warm surface until it just begins to melt.
Fill six of the domes with 2 tablespoons cocoa mix and several mini marshmallows.
Place an unfilled dome on a warm pan until the edge just begins to melt. Immediately place this on a filled dome. These melted edges should line up and create a bond. Repeat for remaining domes.
Decorate as desired.
Place bomb in a mug; heat milk until steaming and slowly pour over the bomb. Wait a few minutes then stir until melted. Note: check the temperature of the drink before serving to children to ensure it isn't overly hot.