For custard, in a small saucepan thoroughly combine 2 tablespoons of the sugar and the cornstarch; add the milk and egg. Cook and stir with a wooden spoon over medium heat just until the mixture begins to bubble. (Do not overcook.) Immediately pour the custard into a small bowl; let mixture cool about 5 minutes.
Whisk sour cream into custard; add vanilla. Cover and chill custard for up to 24 hours.
To serve, divide berries evenly among four goblets or dessert dishes. Spoon chilled custard over berries. (If necessary, thin custard with a little milk before spooning over berries.) Set aside.
For topping, in a heavy small skillet or saucepan heat remaining 2 tablespoons sugar over medium-high heat until sugar begins to melt shaking skillet occasionally to heat evenly (do not stir). Reduce heat to low; cook sugar until melted and golden, stirring as necessary with a wooden spoon. Quickly drizzle caramelized sugar over each custard. Serve immediately.