Using a food processor, pulse together the flour, sugar and salt until combined. Add butter and pulse until mixture becomes coarse meal, about 15 pulses.
In a small bowl, combine egg yolks, water and vanilla extract. Add the egg mixture to food processor and pulse until it starts to clump together. DO NOT pulse until ball is formed. It should be crumbly with large clumps of dough.
Turn dough only a lightly floured surface and form into a ball. Cut in half and flatten each ball slightly to form a disk. Wrap each in plastic wrap and put in fridge for at least 1 hour or up to 3 days.
While pie dough is in fridge, heat a large saucepan over medium-high heat and add the strawberries, blueberries, cornstarch, sugar, lemon juice, vanilla extract and salt. Simmer while stirring constantly for about 8 minutes, or until the mixture starts to thicken. Reduce heat to low and cook for another 2 minutes. Transfer to a heatproof bowl and cool to room temperature.
Remove one disk of dough from the fridge about 10 minutes before you begin rolling. On a lightly floured surface, roll the dough so that it is about 3 inches longer than you pie dish and is between ⅛ inch and ¼ inch in thickness. Press the dough into the bottom and along the sides of the pie plate, being careful not to puncture the dough. Use kitchen scissors to trim the dough overhang to about 1 ½ inches. Fold over and flute the edges.
Arrange a rack in the middle of the oven, set a baking sheet on it, and heat the oven to 375°F. Whisk the egg and water together to make an egg wash. Brush the inside of the pie crust with a thin layer of the egg wash. Pour the cooled filling into the crust.
Unroll the second pie dough on lightly flour the surface and roll to 12 inches in diameter and about ⅛ inch to ¼ inch thick. Use a star cookie cutter and cut out 20-25 stars. Place on top of the berries to form a top crust.
Brush the stars and edges of the pie crust with egg wash and sprinkle with sugar.
Place the pie on the hot baking sheet and bake until the filling is bubbling and crust is golden brown, about 50 to 60 minutes. If the edges of the pie crust are browning too quickly, cover the edges of the pie with aluminum foil. Let cool for at least 3 hours before serving.