In medium microwave-safe bowl, place small chocolate chips.
Microwave at HIGH (100%) 1 minute; stir. If necessary, microwave at HIGH an additional 15 seconds at a time, stirring after each heating, just until chips are melted when stirred.
Add powdered sugar, sour cream and vanilla; beat until smooth.
Add milk, 1/2 teaspoon at a time, for a thinner consistency. About 2-1/2 cups frosting.CREAMY BUTTERCREAM FROSTING:
In small bowl, combine 2 cups powdered sugar, 1/4 cup (1/2 stick) softened butter or margarine, 2 to 3 tablespoons milk and 1/2 teaspoon vanilla extract; beat until smooth and creamy. About 1 cup frosting.