Add water and sugar to a heavy saucepan. Place the over medium heat and stir the ingredients together until the water and sugar are combined.

Allow the sugar mixture to cook for four to six minutes, or until the color changes to a golden brown. Do not stir the sugar while it cooks. When the sugar begins to turn light brown, keep a close eye on the mixture to ensure it does not burn.

When the sugar mixture is a light coppery brown, lift the saucepan and gently swirl the pot over the heat to help ensure even caramelization.

When the sugar is roughly the color of a dark copper penny and the bubbles have reduced in size, pour 1 cup of heavy cream into the pot. Be very careful when adding the cream to the pot because the cream will steam and may bubble over the pot. Reduce the heat to low and whisk together the cream and sugar until well combined.

Remove the saucepan from heat and add 1/2 teaspoon of fleur de sel. Whisk to combine. Once the salt is incorporated into the sauce, add 1/4 cup of sour cream or crème fraîche to the sauce. Whisk until combined and the sauce is smooth. Serve warm, or store in an airtight jar in the refrigerator for several months.