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