Heat a saucepan of water and place a whisk in it.
Wash and dry your hands thoroughly. Combine the water, 1 cup of sugar, and the corn syrup in a medium saucepan. Stir them together with very clean fingers, making sure no lumps of dry sugar remain. Brush down the insides of the pan with a little water, using your hand to feel for any stray granules of sugar.
Cover the saucepan and place it over medium heat for 4 minutes. After 4 minutes, remove the lid, increase the heat to high, and bring to a boil. Do not stir from this point on. Keep an eye on the pan. It will be very bubbly. When stray sugar crystals appear on the side of the pan, brush them down with a wet pastry brush.
As the sugar cooks, the bubbles will get larger. Insert a candy thermometer, and when the temperature reaches 300 degrees F, lower the heat to medium, which will slow the cooking. Continue to cook the sugar until it reaches 350 degrees F. It will be dark brown. Remove the pot from the heat and let it sit for 1 minute, or until the bubbles subside.
Add the cream to the caramel. It will bubble up vigorously, so be careful.
Vigorously whisk in the remaining 1 tablespoon sugar, creme fraiche, lemon juice, and salt. This sauce is now ready to be served warm or cooled to room temperature. It will keep stored airtight in the refrigerator for up to 1 month. When cold, it has the consistency of peanut butter.
Recipe courtesy of Sherry Yard, The Secrets of Baking, Houghton Mifflin, 2003