Line a 9 inch square baking pan with foil and butter the bottom and sides well. Set aside.
In a medium, heavy saucepan, boil the sugar and syrup until thickened slightly. Add the butter and continue to cook to the soft ball stage.(235-245°F)
(See Step 6 for more info on testing for soft/firm ball stage.)
Add 1 c. heavy cream and cook on medium high heat back to the soft ball stage.(235-245°F)
Add one more cup of cream and cook on medium high until caramel colored and to the firm ball stage. (245-250°F)
It is important that the ball remain firm when using the cold water test to keep the caramels from "drooping" or "melting" when finished and cooled. I would cook to the higher temp of 250° to assure the caramels stay firm.
Stir in vanilla, pour into prepared pan, let cool completely. Lift the foil out of the pan and transfer the caramels to a cutting board or marble slab. Cut into approx. 1" square pieces and wrap in waxed paper. Keep cool and dry in airtight container.
Soft ball 235-245* F ----The syrup easily forms a ball while in the cold water, but flattens once removed ----Fudge and fondant
Firm ball 245-250* F ----The syrup is formed into a stable ball, but loses its round shape once pressed ----Caramel candies