In a saucepan, over medium heat, combine the sugar and water. Stir until the sugar dissolves. Stir in the corn syrup. Bring the mixture to a boil and cook until the mixture reaches the soft-ball stage, between 234 and 240 degrees on a candy thermometer. Pour the mixture over a dampened marble slab. Sprinkle it with a little water to prevent a crust from forming and leave to cool for 2 to 3 minutes. Using a triangular scraper work the sugar syrup scraping it from the slab and turning the sides to the center. Alternatively, work the fondant in an electric mixer with a dough hook. Work vigorously particularly when the fondant starts to thicken and become creamy. After 3 to 5 minutes it will suddenly become stiff. Break off one piece of fondant at a time and work it by pinching it hard in your fingers until pliable and smooth. Press all the pieces of pliable fondant together and knead in any flavoring or coloring. Pack into an airtight container and leave in the refrigerator or a cool place at least 1 hour, preferably 1 day to mellow.
Recipe adapted from LaVarenne Pratique, by Anne Willan