Combine 3/4 cup milk and sugar in medium glass bowl.
Cook at MEDIUM 3 1/2 to 4 minutes, or until sugar is dissolved; stir once.
Blend cornstarch with remaining milk until smooth.
Stir into hot mixture.
Cook at MEDIUM 2 to 2 1/2 minutes, or until sauce is thickened; stir once.
Stir in butter and vanilla; chill.