Combine sugar, cream of tartar, salt, and milk in a heavy saucepan. Stir over low heat until sugar dissolves. Wipe crystals occasionally from side of pan. Cook to 238 degrees or until mixture forms soft ball in a cup of tap water. Cool to 220 degrees. Add butter, vanilla, and pecans. Beat until creamy. Drop from a large spoon onto waxed paper. Makes 18-24 pralines. Store individually wrapped in an airtight container.