Line an 8-inch square baking pan with lightly buttered wax paper or aluminum
foil. Set aside.
Combine sugars, salt and milk in large saucepan. Cook over medium heat, stirring
constantly, until sugar dissolves and mixture boils. Continue cooking until a small
amount of mixture forms a soft ball in cold water (or to a temperature of 236 degrees
F / 110 degrees C), stirring frequently after mixture begins to thicken.
Add butter and vanilla extract.
Cool until lukewarm (110 degrees F /
45 degrees C) without stirring.
Beat with heavy wooden spoon until
mixture loses its gloss.