20 ounces bittersweet or semisweet chocolate, finely chopped, divided
1/3 cup sugar
2 tablespoons water
2/3 cup whipping cream
1/4 teaspoon flor blanca (Mexican Sea Salt)
1/2 cup unsweetened cocoa powder
Additional flor blanca
Put 8 ounces chocolate in metal bowl over saucepan of barely simmering water and stir until chocolate is smooth. Remove chocolate. Combine sugar and 2 tablespoons water in small saucepan. Stir over medium heat until sugar dissolves, occasionally brushing sides of pan with wet pastry brush. Increase heat; boil until syrup is deep amber color, brushing down sides and swirling pan occasionally, about 4 minutes. Add cream (mixture will bubble). Stir over very low heat until caramel is smooth. Mix caramel and 1/4 teaspoon flor blanca into melted chocolate. Chill until truffle filling is firm, at least 3 hours.
Place cocoa in bowl. Using 1 Tablespoon truffle filling for each truffle, roll into balls, then roll in cocoa. Arrange on baking sheet. Cover and chill overnight.
Line a 13x9x2-inch baking sheet with parchment paper. Place remaining 12 ounces chocolate in medium metal bowl over saucepan of barely simmering water and stir until chocolate is melted and smooth and thermometer inserted into chocolate registers 115°F. Remove bowl from over water. Working quickly, submerge 1 truffle in melted chocolate. Using fork, lift out truffle and tap fork against side of bowl to allow excess coating to drip off. Transfer truffle to prepared sheet. Repeat with remaining truffles. Sprinkle truffles lightly with additional flor blanca. Let stand until coating sets, at least 1 hour. (Can be made 1 week ahead. Cover and chill. Bring to room temperature before serving.)