Mix sugar, syrup, salt, & 1 cup of cream in a large pan (non-stick is best). Cook slowly to prevent scorching! Using a candy thermometer, cook to 234°; slowly add remaining cream and cook to 234° again. Remove from flame & add butter. When butter is melted, return to fire and cook to 248°. Add vanilla; if using nuts, sprinkle nuts on buttered pan 9 x 13 pan (for thicker caramels) or 10 x 15 pan (for thinner caramels), then pour caramel into pan. When cool, cut into squares and wrap in waxed paper.