In a heavy saucepan, mix the sugar, corn syrup, water and salt. Cook to the hard ball stage using a candy thermometer. (250 to 266°F. The syrup holds its ball shape and deforms only slightly with very firm pressure, but remains sticky).
Remove the pan from heat and let it sit while you beat the egg whites very stiff and dry with an electric mixer. Beat at medium speed while pouring the hot syrup into the egg whites. The mixture must be poured slowly in a very fine stream. Continue beating and add the butter and vanilla. Beat until the candy will hold its shape, and then add the frozen pecans.
Spoon a little of the mixture onto wax paper to see if it will hold its shape. If it doesn't, continue to beat by hand until it will drop by spoonfuls onto wax paper.