Preheat oven to 350°F.
Lightly coat 24 to 30, 2 1/2-inch muffin cups with nonstick cooking spray; set aside.
In a large mixing bowl, stir together the cake mix, pie filling and 3 eggs until moistened.
Remove 1 cup of the batter and set aside. Divide remaining batter among prepared muffin cups filling each about half full.
In a medium bowl, whisk together sour cream, 1 egg, sugar, flour and vanilla until smooth. Spoon small mounds of reserved batter over sour cream mixture in each cup (batter will not cover sour cream layer).
Bake for 25 minutes or until tops spring back when lightly touched.
Remove and cool in pans on wire racks for 15 minutes. Loosen edges and remove from pans. Cool completely. Chill within 2 hours.