Preheat oven to 425°. Gently toss together first 5 ingredients.
Unroll 2 piecrusts on a lightly floured surface. Cut crusts into 10 rounds using a 4-inch round cutter. Fit into 10 lightly greased 3-inch fluted tart pans; press into edges. Spoon berry mixture into pans; dot with butter.
Unroll remaining 2 piecrusts on lightly floured surface. Cut into 10 (4-inch) rounds. Cut 3 (1-inch) stars from top of each round. Place rounds over filling in each tart pan; press into fluted edges to seal. Top each tart with 3 stars, leaving cutouts open for steam to escape. Freeze pies on an aluminum foil-lined 15- x 10-inch jelly-roll pan 10 minutes.
Meanwhile, whisk together egg yolk and 1 Tbsp. water. Brush tops of pies with egg mixture; sprinkle with 1 Tbsp. sugar.
Bake pies on jelly-roll pan at 425° for 24 to 26 minutes or until bubbly and golden brown.