Melt semisweet chocolate over hot, not simmering water in double boiler. Set aside to cool to lukewarm. In lg bowl, cream butter with mixer. Beat in melted chocolate. Beat in vanilla, flour, sugar, cocoa, and salt. Wrap and chill dough 30 mins or until f irm enough to roll into balls. Preheat oven 350 F. Roll dough into 1" balls, then roll each ball into a thick log. Place on ungreased cookie sheet; press dough into a 1/4" thickness with the tines of a fork, keeping cookies oval in shape. Bake 8-10 mins or until just set. Do not overbake. Transfer to wire rack. Dipping Cookies: In sm bowl set over a saucepan of hot water, melt white chocolate with 1/4c cream, stir until smooth. Keep mixture ove rhot water so it will be liquid for dippig. In another sm b owl set over a saucepan of hot wtaer melt semisweet chocolate with the remaining 1/4c cream; stir until smooth. Keep warm. Dip one end of a cookie in the white chocolate and the other end in the dark chocolate; return to cooling ! racks so the chocolate can set. Repeat wtih remaining cookies.