Combine clam juice and wine in heavy small nonaluminum saucepan. Boil until reduced to 1/3 cup, about 9 minutes. Reduce heat to medium. Whisk in creme fraiche. Boil until reduced to 1 cup, about 5 minutes. Remove from heat. Stir in dill. Season with salt and pepper. If creme fraiche is unavailable, heat 1 cup whipping cream to lukewarm (85°F). Remove from heat and mix in 2 tablespoons buttermilk. Cover and let stand in warm draft-free area until slightly thickened (24 to 48 hours, depending on temperature of room). Refrigerate until ready to use.