1/3 cup unsalted butted, softened
1/3 cup light corn syrup
1 tsp. vanilla extract
1/2 tsp. salt
1 pound confectioners' sugar, sifted

Cream butter slightly in a large mixing bowl; blend in corn syrup, vanilla and salt. Add confectioners' sugar all at once and mix with a wooden spoon then knead with hands. Turn onto a smooth surface and continue to knead until mixture is well blended and smooth. Store in a cool place. Shape as desired.

Variations:

Mint: Substitute 1/2 tsp. peppermint extract for the vanilla extract, shape into balls or roll thin and cut into desired shapes using tiny cookie cutters.

Orange or Lemon: Substitute 1 tsp. orange extract or 1 tsp. lemon extract for the vanilla extract. Tint a delicate orange or yellow. Shape as desired.

Peanut Squares: While kneading the fondant, mix in 3/4 cup coarsely chopped unsalted peanuts. Roll out or pat to 1/2" thickness. Cut into squares.

Almond Diamonds: Substitute 1/2 tsp. almond extract for the vanilla extract. While kneading the fondant, mix in 1/2 cup toasted, blanched almonds. Roll out or pat to 1/2" thickness. Cut into diamonds.

Mocha Logs: Add 1 tsp. instant coffee to confectioners' sugar before adding to the butter mixture. Shape fondant into rolls 2" long and 1/2" thick. Roll in chocolate sprinkles.

Chocolate Creams: Add 1/4 cup cocoa powder to sugar before mixing into the butter mixture. Shape into balls or roll thin and cut into desired shapes.

Fruit Squares: Substitute 1/2 to 1 tsp. rum extract for the vanilla extract. While kneading add 1/2 cup finely chopped, mixed candied fruit. Roll out to 1/2-inch thickness. Cut into squares.

Circus Balls: Shape fondant into 1/2-inch balls. Roll in multi-colored sprinkles

Additional variations can be found on http://www.baking911.com/asksarahbb/index.php?showtopic=2167