Line either an 8 x 8 baking dish or a 9 x 5 loaf pan with foil. Butter the foil. Note: An 8 x 8-inch pan will give a thinner candy; a 9 x 5-inch loaf pan will give a thicker candy.
Melt butter in large microwave-safe bowl. (I used a large Pyrex bowl.)
Stir in Karo syrup, both sugars, and sweetened condensed milk. Mix until most of the sugar is dissolved.
Microwave on high for 3 minutes for softer caramels, or 3 1/2 minutes for firmer caramels. Using hotpads, carefully remove bowl from microwave. Be careful, it is very hot!
Stir down and scrape sides of bowl.
Microwave on high 3 minutes (for softer caramels or 3 1/2 minutes (firmer caramels). (This will be soft ball stage.) Using hot pads, remove from microwave and stir in nuts (if using) and vanilla.
Pour immediately into buttered dish and refrigerate until set, about 1 hour.
Remove from refrigerator and allow to come to room temperature before cutting. The caramels can be left plain or decorated by dipping them in chocolate and then in chopped nuts, sprinkles, etc. Wrap each candy piece in wax paper.