Combine sugar, light corn syrup, cream of tartar, and water in a 2-quart saucepan over high heat and give it a stir. Allow the sugar to dissolve.
When the sugar has dissolved, reach for your digital instant-read thermometer. Pretty soon, the mixture will reach a boil and the temperature will start increasing.
Once the thermometer hits 230°F, turn the heat down to medium. Once the temperature reaches 300°F, after about 5 minutes, give the mixture a stir.
Once you get to 340°F, turn the heat off and remove the thermometer. Be careful, it’ll be hot.
Pour in the heavy creamStand back because it’ll bubble. and just wait. The bubbling is just the moisture cooking out of the cream. As soon as you can safely approach the cooktop, return the heat to medium and continue cooking for 3 minutes.
Remove from heat and let it cool thoroughly before containment. In the fridge, the sauce will last about 1 month.