Place the can of condensed milk in a pot and cover it completely with water. Cover the pot with a lid and bring the water to a boil. Once boiling, reduce the heat so the water simmers gently. Let it simmer for 3 hours. IMPORTANT: Check occasionally to make sure the water hasn't evaporated. The can must stay fully submerged, or it could explode. If the water level drops, add more so the can stays covered at all times.

Fish the can out with a fork or tongs and let it cool for at least 1 hour. Don't open the can while it's cooling! Transfer the cooled caramel to a piping bag**.

Melt and temper two-thirds of the dark chocolate. Pour the melted chocolate into all the cavities of the chocolate mold. Turn the mold upside down and tap out the excess chocolate onto a piece of parchment paper. Scrape any excess chocolate off the top of the mold. Let the chocolate shells set.

Pipe caramel into the chocolate shells. Melt and temper the remaining third of the dark chocolate. Pour the melted chocolate over the caramel-filled cavities until they're completely filled. Scrape any excess chocolate off the top of the mold. Let the chocolate lids set.

Tap the filled chocolates out of the mold.