Add sugar, water, cream of tartar, lemon juice, and vanilla (if using) to a heavy-bottom saucepan. Mix and then turn the heat to medium.
When the sugar is dissolved and bubbles up, turn the heat to a simmer and add a candy thermometer to the pan.
After about 15 to 25 minutes (or more), when it reaches the jelly stage, or between 220 and 230 F (106 C), remove it from the heat and put the pot in a bowl of cold water to cool it down quickly.
When it cools, store it in a glass jar in the pantry/cupboard at room temperature for up to 3 months.
