Preheat the oven to 350 degrees F. Line 10 muffin cups with muffin paper cups.
In a food processor, combine ricotta cheese, cottage cheese and sugar; puree until smooth. Beat in egg. Blend in sour cream, cornstarch and vanilla until well mixed. Divide batter among muffin cups.
Set muffin tin in larger pan; pour in enough hot water to come half way up sides. Bake 30 to 35 minutes or until tester inserted in center comes out clean. Remove from water bath; cool on wire rack. Chill.