Melt the chocolate using one of the methods below. (Melt each type in its own bowl)
Add one tablespoon of milk chocolate to 6 of the silicone cavities and use a pastry brush or the back of a small spoon to spread the chocolate paying special attention to make sure it reaches the edges.
Repeat filling 6 cavities with the white chocolate then place the tray in the freezer for 5-10 minutes to allow the chocolate to set.
Remove the tray and repeat the process of brushing chocolate in the moulds, this time using 1-2 teaspoons per cavity. You just need to add enough to cover any spots where you see the silicone through the chocolate and to reinforce the edge. Place the tray back in the freezer to set.
Once the shells are fully set remove them from the freezer and fill 6 halves with one tablespoon of cocoa mix and 1-2 teaspoons of espresso powder depending on how strong you want them.
Remove the empty shells from the silicone mould.
Fill a mug with boiling water and place a small plate on top.
Working one at a time, place an empty shell face down on the warm plate. When you see the edge begin to melt remove it from the plate and immediately place it on top of a filled half. Press and hold gently for 2-3 seconds to fuse the halves together. Repeat with remaining shells.
Place the tray back in the freezer for 5-10 minutes to make sure the shells are fused, then remove the tray from the freezer and pop the chocolate bombs out of the mould.
Decorate as desired with melted chocolate and candy sprinkles.