Pour water to depth of 1 inch into bottom of a double boiler over medium-high heat, and bring to a boil. Reduce heat to medium-low, and simmer; place chocolate and salt in top of double boiler over simmering water. Cook, stirring constantly, 2 to 3 minutes or until melted. Add sweetened condensed milk; cook, stirring constantly, 1 to 2 minutes or until blended and smooth. Remove from heat; add butter and vanilla, and stir 4 to 5 minutes or until smooth. Let cool to room temperature (about 45 minutes). Transfer to a bowl. Gradually add cream to chocolate mixture, and beat with an electric mixer at high speed 2 to 3 minutes or until smooth and the consistency of a thick buttercream frosting. Use immediately.
For best results, use premium chocolate morsels. We tested with Ghirardelli Chocolate Premium Semi-Sweet Baking Chips.