Combine cornstarch and milk in a small bowl. Place cornstarch mixture, yogurt, mint, rind, pepper, salt, feta, and Gorgonzola in a food processor; pulse to blend. With processor on, add egg and egg white; process until creamy.
Spoon 1/4 cup yogurt mixture into each of 12 small ramekins or custard cups. Bake at 325° for 25 minutes or until tarts are almost set but still slightly wobbly. Turn off heat; leave in oven 5 minutes. Remove from oven; cool 15 minutes. Top each tart with a mint leaf.