For custard, in a heavy medium saucepan combine sugar and cornstarch. Stir in milk. Cook and stir over medium heat until mixture is thickened and bubbly. Cook and stir for 2 minutes more. Remove from heat. Gradually stir about half of the hot mixture into the beaten egg. Return all of the egg mixture to the saucepan. Cook until nearly bubbly, but do not boil. Immediately pour custard into a bowl; stir in sour cream and Marsala. Cover the surface with plastic wrap. Chill for 2 to 24 hours.
To serve, divide the berries evenly among 4 dessert dishes. Spoon custard evenly over the berries. If desired, sprinkle with cinnamon or nutmeg and garnish with mint. Serve immediately. Makes 4 servings.