I still make my own caramel popcorn around this time of the year. Hopefully this is the recipe you're looking for though I don't know if it came from Tupperware or not............
18 cups of popped corn or enough to fill a large roasting pan, grease the pan first with either butter or Pam.
1 cup of butter ( I use 1.5 cups or 3 cubes)
2 cups of brown sugar (I use 3 cups)
1/4 c. of white syrup ( I use 1/3 cup)
1 TB salt
1 TB Baking Soda
Combine butter, sugar and syrup in a pan, bring to a boil, boil for 5 minutes and then add the salt and soda. This will make the mixture foam up. Quickly pour over your popped corn. Bake at 250, every 15 minutes stir to insure the caramel covers all the popcorn. Let cool and enjoy.
I use more of all ingredients as I like lots of caramel on my popcorn. Just a personal preference.