Line an 8-inch square baking pan with two pieces of aluminum foil, letting the foil extend over all sides by about 2 inches.
Melt the chocolate with the cream and salt in a large heatproof bowl set over a saucepan with about 1 1/2 inches of nearly simmering water, whisking until smooth. Remove the bowl from the heat and whisk in the vanilla.
Pour the chocolate mixture into the prepared pan, spread it evenly with a rubber spatula and let cool. Refrigerate, tightly covered, for about 3 hours or until firm.
Invert the foil on a cutting board and carefully remove the foil. Dip a sharp heavy knife into hot water, wipe it thoroughly dry, and cut the truffles into 36 squares, rewarming the knife as necessary. If the truffle mixture gets too soft, return it to the refrigerator to firm. Arrange the truffles in a single layer on a platter. Refrigerate, uncovered, for 1 hour.
Place the cacao in a small shallow bowl. Toss the truffles a few at a time in the cacao to coat, then shake them to remove any excess cacao. Transfer to another plate. Refrigerate the truffles, covered, for at least 1 hour before serving. If necessary, toss them again in cacao to coat. The truffles can be refrigerated for up to 3 days. Store them layered between sheets of waxed paper in a sealed container. Serve at room temperature.