Note: Semisweet chocolate should be coarsely chopped. * Unsalted butter should be cut into small pieces. In a double boiler, melt the chocolate and butter over simmering water. Remove from the heat and stir in the vanilla. In a small bowl, gradually whisk the warm chocolate mixture into the egg yolks. Cover and refrigerate until firm, about 1 hour. Shape level tablespoons of the chocolate mixture into balls and then roll them in the cocoa. Refrigerate the truffles.