How to Make It
Combine first 3 ingredients in a large bowl. Mix in 1 cup cold butter with a pastry blender or two forks until mixture resembles coarse meal. Whisk together egg yolk and 3 tablespoons milk; pour into flour mixture, stirring just until dough starts to form a ball. Press together with lightly floured hands, and form into a flat round. Cover with plastic wrap; place in refrigerator at least 1 hour or overnight.
Preheat oven to 375°.
Combine blueberries, raspberries, 1/3 cup sugar, and next 4 ingredients in a large bowl.
Remove dough from refrigerator; divide evenly into 8 pieces. Roll each piece on a lightly floured surface into about 6-inch rounds; place on 2 lightly greased large baking sheets. Scoop 1/4 cup berry mixture into center of each round; top each with butter. Fold about 1 inch of dough over fruit all the way around to form a rim.
Combine egg and milk; brush outside of each tart with the egg wash. Sprinkle tarts with about 1 to 2 tablespoons sugar. Bake at 375° for 40 to 45 minutes or until golden brown, rotating baking sheet halfway through baking. Let cool on pan 5 minutes. Remove to a wire rack; let cool. Serve with lightly sweetened whipped cream or vanilla ice cream.