In a heavy pan, melt 3 cubes (1½ c.) butter (now & then I cheat & substitute a margarine for one of the butters) then add:
1 can sweetened condensed milk
1½ cup sugar
1½ cup brown sugar
1½ cup light Karo syrup
Stir everything together over medium heat until it’s blended together. Then, cook over medium to med.-high heat, STIRRING CONSTANTLY, until it reaches about 235º or 236º. (If you want them harder, go another degree or two…softer is about 234º. You may need to make them a couple of times before you get it just right!).
Pour into a lightly, lightly buttered (or spray and then wipe down) 9 x 13 pan. If you put it in a metal pan, ONLY CUT CARAMELS WITH A PLASTIC KNIFE to prevent getting metal shavings in the caramels. Update: Line a pan with heavy aluminum foil; spray foil. If all goes well, you can lift out the foil, peel it off, and then put caramels on a cutting board and cut!