Meanwhile, stir unflavoured gelatine into 1-1/2 cups milk; let stand 5 min. or until gelatine is softened. Bring 2 cups of the remaining milk just to boil in large saucepan on medium heat. Add sugar and unflavoured gelatine mixture; cook and stir 3 to 5 min. or until sugar is dissolved. Remove from heat; cool 5 min. Add sour cream, vanilla and remaining milk; whisk until blended. Refrigerate 30 min. or until cooled, but not firm.