In a large enough pot to fit all of the ingredients, whisk together the sugar, water, and cream of tartar.
Heat the mixture on low heat.Once the water warms a bit, whisk everything again until the sugar dissolves.
Once the sugar dissolves, continue to heat the mixture on low heat for 35-40 minutes without stirring at all. Periodically check the temperature with a candy thermometer and remove the mixture from heat once it reaches 235-240F (112-116C).
Allow the mixture to cool and transfer to an airtight container and store in the refrigerator.