In a 3 quart saucepan combine sugar, brown sugar and syrup. Cook rapidly over medium high heat until candy thermometer reaches 242 degrees or firm ball stage
Add butter and evaporated milk gradually, so that mixture does not stop boiling at any time. Cook over medium heat until mixture reaches 242 degrees or firm ball scale, stirring constantly, about 25 minutes.
Remove from heat. Add vanilla and nuts. Combine well. Let stand until mixture stops bubbling.
Pour into buttered 8 or 9 inch square pan.
Cool at room temperature, then chill in refrigerator until caramel hardens.
When caramel is ready to cut, set pan in warm water to loosen from bottom and sides of pan. Turn into cutting board. Cut into small squares with a sharp knife, using a saw-like motion.
Makes 2 pounds.