Note: making caramel is something that needs your undivided attention. Make sure you have all your ingredients gathered before starting and that you are not distracted. Leaving the caramel alone at any point can cause it to burn.
Place sugar in a heavy-bottomed medium sauce pot. Make sure the sides of the pot come up a little bit (that it’s not a shallow pan) because it will boil up when you add the cream later.
Place the pan over medium-low heat. Cook stirring often, but being careful not to spread the sugar up the sides of the pan too much. The sugar will become a little bit liquidy, form clumps, then finally melt into a light amber liquid. Keep stirring until the liquid is amber in color. This will take about 7-12 minutes, depending on your pot and the heat of your stove. DO NOT LEAVE THE SUGAR ALONE, it will burn fast.
Once the liquid turns amber, add the butter and stir until it’s melted. Add half of the heavy whipping cream, stir, then add the rest. It will bubble up as you add it to the mixture. Turn off the heat and then stir in the, cinnamon, vanilla, and salt, if using. Stir until smooth. You may need to whisk it to get out all the clumps.
Pour the caramel carefully into a jar and let cool to room temperature. It will thicken as it cools. Cover and store in the refrigerator. When ready to use, remove the lid and heat in 30 second increments until the caramel is melted and smooth and the desired consistency (about 60-90 seconds for the just-made texture). Will keep for 2 weeks in the refrigerator.