Preparation - Combine butter, sugar, brown sugar and Karo syrup in a saucepan. Stir the ingredients together, then bring to a boil over medium heat. Reduce heat to low and cook for 10 minutes, stirring constantly. Remove from heat, and add the condensed milk. Mix well. Return to heat and cook until the temperature on a candy thermometer reaches 245º, or the firm ball stage. (At this point a tiny amount dropped into ice water forms a ball that stays firm and pliable but is still sticky.) Remove from heat and stir in vanilla and nuts(if using) Pour the mixture into a buttered 9" x 13" pan. Cool completely before cutting into 1" to 1-1/2" squares. Wrap individual caramel squares in waxed paper. Store in an air tight container.
