Add the sugar and water to a 3-quart heavy-bottomed pot. Stir just enough to combine them in an even layer, keeping the mixture off the sides of the pot.

Cook over medium heat until the mixture changes from cloudy to clear and begins to bubble. Do not stir once it starts bubbling.

Continue cooking without stirring until the syrup thickens and turns deep gold, like honey. Watch closely so it does not burn. If sugar clumps around the edge of the pot, gently brush down the sides with a spatula.

Carefully add the butter and whisk until it melts completely. The caramel will bubble rapidly.

Remove the pot from the heat. Slowly pour in the heavy cream, whisking continuously until it is incorporated.

Whisk in the vanilla and salt. Let the sauce cool in the pot for 10 minutes, then transfer it to a heatproof lidded jar and refrigerate.