Place sugar and water into a medium pot, stir to combine, but do not stir from this point forward. Cook over high heat, washing down the sides of the pot with a pastry brush dipped in water as needed to prevent crystals.
Cook until desired color of caramel is reached (amber) and immediately remove from heat. Very slowly pour in (room temperature) heavy cream while whisking quickly. The mixture will bubble up (a lot) and boil. Add (room temperature) butter and whisk in.
If you want to make this a flavored caramel, add your flavoring at this point. Whisk it in (start with less than you think you'll need), dip a spoon into the caramel, let it cool, taste it, add more as needed.
Return to heat and simmer gently for 1-2 minutes while whisking constantly.
Remove from heat and let cool to room temperature (about 2 hours). Transfer to a container and allow to thicken.
Store at room temperature for a day or two, in the fridge for 2 weeks, or the freezer for 3 months.