English Toffee

IN A HEAVY 3 TO 4 QUART SAUCE PAN, OVER MEDIUM HEAT, MELT BUTTER. STIR IN SUGAR, WATER, AND SALT. COOK AND STIR UNTIL SUGAR IS DISSOLVED. INSERT CANDY THERMOMETER AND CONTINUE TO COOK UNTIL THERMOMETER REACHES 290 DEG F. POUR MIXTURE ONTO GREASED 10 X 15 JELLY ROLL PAN AND SPREAD EVENLY TO EDGES OF PAN. COOL AT ROOM TEMPERATURE. IN A DOUBLE BOILER, OVER SIMMERING WATER, MELT CHOCOLATE. SPREAD HALF THE CHOCOLATE OVER THE TOFFEE. SPRINKLE WITH HALF THE NUTS. WHEN COOL, T