To make a simple homemade creme fraiche, mix 1 cup of heavy cream with 2 tablespoons of buttermilk in a small bowl or glass jar. Cover with a lid or kitchen towel, and leave the container on the counter for up to 16 hours, or until the mixture thickens.
To achieve a lighter flavor, reduce the buttermilk to 1 tablespoon, and increase the cream to 2 cups. For the best results, use a cream that is pasteurized rather than sterilized or ultra-pasteurized. Speed the thickening process by heating the cream and buttermilk mixture over low heat until it reaches 85 degrees. Creme fraiche sets best when placed in a warm location that is sheltered from cool breezes, such as the kitchen counter or the top of the refrigerator.
After the creme fraiche thickens to your desired consistency, stir it well and store it in the refrigerator. Allow the flavor to set by letting the creme fraiche rest for 24 hours.
Change the flavor of the creme fraiche by stirring in finely chopped fresh herbs to taste. For use in sweet dishes, mix in small amounts of orange juice or lemon juice. Creme fraiche also pairs well with vanilla, chives, tarragon and sage. Store unused creme fraiche in the refrigerator for a maximum of two weeks.