Make the sauce: Combine the sugar and water in a small, heavy saucepan. Bring to a boil. Continue to boil the mixture until it turns a light amber color. Do not stir. If part of the syrup is turning darker than the rest, gently tilt the pan to even out the cooking.
When the syrup is a uniform amber color, stir in the heavy cream with a long-handled wooden spoon. When the bubbling has subsided, remove the pot from the heat and stir in the remaining butter and salt until the butter is melted. Transfer to a large heatproof measuring cup and let cool to warm before using.