Preheat oven to 425 degrees. Stir together blueberries, sugar, cornstarch, 1 teaspoon lemon zest, juice of half the lemon, vanilla, and salt in a bowl. Set it aside once it's all combined. Unroll the two discs of dough. Using a 5-inch round pastry cutter (or you can trace a knife around a 5-inch bowl) cut three rounds of pastry from each disc. Re-roll the scraps and cut 2 more. Place them on 2 baking sheets lined with a baking mat or parchment (4 rounds each pan). Evenly distribute the blueberry mixture between the six discs (approximately 1/4 cup per round). Gently fold the over the edges of each crust, folding the dough in on itself to create a small rim of crust. Make an egg wash by beating together the egg and water. Brush edges of each pie with the egg wash and sprinkle the crust with sugar. Bake for 15 minutes until golden and bubbly. Remove from the oven and allow pies to sit on the pan (they will leak a little juice while baking, but this is fine!) for 5 minutes. Remove to a platter and allow to cool.Serve warm or at room temperature with a dollop of whipped cream. Divine!