Combine first 4 ingredients in the top of a double boiler. Cook over simmering water 38 minutes or until mixture thickens and coats 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; add 1/4 cup strawberries and 1/4 cup raspberries to each glass. Garnish with mint, if desired.