Preheat oven to 425°. Crush half the blueberries. Sift together sugar and cornstarch. Add whole and crushed blueberries; toss until berries are well coated. Set aside., On a lightly floured surface, unroll crusts. Cut out six 4-1/2-in. circles; press circles onto bottoms and up sides of greased muffin cups. Evenly spoon in blueberry mixture. Cut out six 2-in. circles from remaining crust; place over filling. Brush with yolk., Bake until crust is golden and filling bubbles, 13-17 minutes. Cool in pans 10 minutes; run a knife around sides of muffin cups and remove tarts to a serving plate.