Combine sour cream and whipping cream in a small mixer bowl. Refrigerate with beaters until well chilled. Beat creams until frothy. Gradually add sugar, vanilla and salt, beating until quite stiff. Refrigerate until serving time. Will maintain consistency for 1 to 2 hours. Makes about 2 cups.
Variation: Orange Creme Fraiche: Add 1 tablespoon orange juice, 1 teaspoon lemon juice and 1/2 teaspoon grated orange peel.