For the Cornmeal Tart Dough:
1. Combine the flour, cornmeal, and salt in a bowl. Whisk to combine and set aside.
2. Cream together the butter and sugar. Stir in the yolks, one at a time, making sure each one is well combined before adding the next. Add the vanilla and mix well. Sift the dry ingredients over the wet ingredients and stir to combine until the dough forms a ball.
3. Divide the dough in two and pat into flat disks. Wrap each disk tightly in plastic wrap and chill or freeze until ready to use.
For the Blueberry Cornmeal Crostadas:
1. Remove the dough from the refrigerator about 30 minutes before rolling.
2. Combine the blueberries, sugar, flour, lemon zest, and lemon juice in a large bowl. Toss together gently and set aside.
3. When the dough is malleable but still firm, cut each disk into quarters. Working quickly, use your hands to form a ball with one piece of dough. Flatten the ball and place between 2 pieces of plastic wrap. Roll out into a circle 5 to 6 inches in diameter and about 1/8-inch thick. Leave it in the plastic wrap and place the rolled dough on a plate. Transfer to the refrigerator. Continue to roll out the remaining dough putting each in the refrigerator as you finish. Chill until firm but still workable.
4. Line a large baking sheet with parchment paper. Place a circle of dough on the baking sheet, leaving room for the other crostatas. Spoon about ½ cup of the berry mixture onto the circle, leaving at least a 1-inch border. Bring the edges of the dough over the top to envelop the berries. Using your hands, fold in the edges, pressing gently and patching any holes. Cup your hands over the tart and press down firmly to shape. Repeat with remaining dough and berries. Chill for about 20 minutes before baking. While the tarts are chilling, preheat the oven to 375° F.
5. Bake until the crust turns golden brown and the fruit is bubbling, about 25 minutes. Let set for 5 to 10 minutes before transferring from baking sheet to serving plates. Serve warm or at room temperature with vanilla ice cream or Homemade Crème Fraîche.