Combine milk, sugar, and cornstarch in a 2-cup glass measure.
Microwave, uncovered, at HIGH 2 minutes or until thickened, stirring every 30 seconds.
Gradually stir about one-fourth of hot milk mixture into beaten egg; add to remaining milk mixture, stirring constantly with a wire whisk.
Stir in brandy and vanilla extract.
Microwave at MEDIUM (50% power) 1 minute or until thickened, stirring after 30 seconds.
Cover and chill thoroughly.
Combine raspberries, strawberries, and blueberries.
Spoon 3/4 cup berry mixture into each of 4 individual dessert dishes; top evenly with custard sauce.