In a double boiler or heatproof bowl set over a pot of simmering water, heat the chocolate and the sweetened condensed milk over medium low heat. Stir often until the chocolate is smooth, melted, and thoroughly combined with the milk. The mixture will be quite thick. Turn the heat off and stir in vanilla.
Allow the bowl to cool slightly, then cover and refrigerate for 1.5 hours.
Using a cookie dough scoop or spoon, roll the truffle mixture into tablespoon sized balls. If balls are too soft or sticky after this process you may want to pop them in the freezer for about 15 minutes before dipping.
Meanwhile, in a shallow medium sized bowl, melt the dipping chocolate in the microwave or in a double boiler on the stove top.
Using a slotted spoon or fork, dip each ball into the chocolate mixture and coat completely. Use a chopstick, toothpick, or knife to scrape excess chocolate off the bottoms. Place on waxed paper. Allow to dry for a few seconds before sprinkling with sea salt. Truffles keep well refrigerated or at room temperature for up to 1 week, but may develop a white cast. Allow to come to room temperature before serving.