In a large saucepan, melt the butter and sauté the shallots and leek for 3 to 5 minutes or until translucent. Add the apples, sorrel, and water and simmer for 25 minutes.
In a blender or food processor, puree the apple mixture. Return the mixture to the saucepan and stir in the buttermilk and yogurt. Bring to a boil, then remove from heat immediately or separation will occur. Add the salt and black pepper and chill for 2 to 3 hours.