Combine first 4 ingredients in the top of a double boiler.
Cook over simmering water 38 minutes or until mixture thickens and coats back of a metal spoon, stirring constantly.
Pour custard into a bowl, and stir in vanilla.
Let custard sauce cool to room temperature.
Cover and chill.
Spoon about 1/3 cup custard sauce into each of 6 stemmed glasses; top each with 1/4 cup strawberries and 1/4 cup raspberries.
Garnish with mint, if desired.