In a medium saucepan, combine the 1 cup granulated sugar and ¼ cup water and bring to a boil, whisking until the sugar melts. Turn to medium heat and slowly boil without whisking until a deep amber caramel forms, about 6 minutes. Remove the saucepan from the heat and carefully whisk in the 4 tablespoons unsalted butter; it will foam up. Whisk in the ¾ cup heavy cream (it will foam up again) and 1 teaspoon fine sea salt.
Let the caramel cool to room temperature and store in a glass container for up to 2 weeks in the fridge.
Notes
Use fresh cream. The carton of cream you pick up at the store should just be a few days old, so that the caramel sauce tastes the best it can. No old cream, please!
Add the cream slowly. Pour in the cream slowly while you're whisking. It will foam and bubble up - this is normal! Just keep whisking until a smooth sauce forms.
Make sure the sugar is fully dissolved. Cook the sugar-water mixture, whisking, until the sugar dissolves and the mixture looks clear. This will ensure the smoothest sauce. Then, move on to the next step.
Use a thermometer. When I'm in a rush, I go by my eyes and a timer. But I also find it helpful to use a candy thermometer. What temperature should it reach? Read on ...
Caramel sauce temperatures: For a darker, more amber-colored sauce, cook the sugar mixture until it reaches 320-340°F on a thermometer. For a thinner, more pourable sauce (like what I've shown in my photos), cook it to a bit lower of a temperature at 220-230°F.
Don't go above 350°F. This is the temp at which caramel burns! Avoid at all costs!
If it breaks ... A trick I use if my sauce "breaks" when I add the butter is to return it to the stove on low heat where I whisk until the butter melts and is incorporated into the caramel.