In a rather deep saucepan, heat butter, brown sugar, and light corn syrup over medium to medium-high heat, stirring constantly. Bring the mixture to a boil and continue cooking for 3 minutes, stirring constantly. Try to keep mixture off sides of pan.
Add the cream and salt and continue stirring until it begins to boil. Remove from heat, add the vanilla, and cool for 15 minutes or so.
Pour into the bowl of your mixer if it is metal or a heat proof bowl. You can also use a hand mixer. Pour carefully as this is very hot.
Add the confectioners sugar to the hot mixture bit by bit, mixing at medium speed until smooth.
The mixture will thicken as the powdered sugar is added.
Add a small amount of very hot water (one teaspoon or tablespoon at a time) as needed until the frosting reaches a good consistency for spreading. Makes about 4-4.5 cups of frosting. When not using the frosting, place plastic wrap over the bowl.