Melt bacon grease in a two-quart saucepot over medium heat. Add leeks, onion, garlic and salt. Cook until the vegetables become soft. Add potatoes, chicken broth and bay leaf. Cover pot and cook on a low simmer until potatoes become tender. Allow mixture to cool for 15 minutes. Combine whipping cream, sour cream, egg yolk and white pepper. Place the soup base in a blender and puree. Add cream mixture and blend until smooth. Chill completely before serving.
Yield: three quarts