To make the pastry: Whisk together the flour, salt, and baking powder. Add the butter, working it in to make a coarse/crumbly mixture. Leave most of the butter in large, pea-sized pieces., Add the sour cream, and stir until the mixture starts to come together in chunks. Turn it out onto a floured work surface, and bring it together with a few quick kneads., Pat the dough into a rough log, and roll it into an 8" x 10" rectangle. Dust both sides of the dough with flour, and starting with a shorter end, fold it in three like a business letter., Flip the dough over, give it a 90° turn on your work surface, and roll it again into an 8" x 10" rectangle. Fold it in three again., Wrap the dough, and chill for at least 30 minutes before using., To make the filling: If you're using fresh berries, rinse and drain well. Place fresh or frozen berries in a saucepan. Whisk the cornstarch or ClearJel with the sugar, and pour over the berries. Add the salt and lemon juice, stirring to combine., Place the saucepan on a burner set to medium-high heat and cook, stirring, until the small amount of liquid in the bottom of the pan comes to a simmer. Reduce the heat to medium and continue to cook, stirring frequently, until the mixture starts to thicken, about 5 minutes. Transfer the cooked berries to a bowl and let cool to room temperature. It's fine to make the filling ahead of time, and refrigerate until you're ready to use it., Preheat the oven to 425°F; place a rack on the middle shelf. Line a baking sheet with parchment paper., To assemble the pies: Roll the dough into a 14" x 14" square. With a straight edge and pastry wheel, or a 3 1/2" square cutter, cut out sixteen 3 1/2" squares., Divide the filling among eight of the squares, using about a heaping tablespoon for each; a slightly heaped tablespoon cookie scoop works well here. Brush some of the beaten egg along the edges of each filled square., Use a knife to cut a vent into each of the remaining eight squares; or use a decorative cutter of your choice., Top each filled square with a vented square, and press along the edges with the tines of a fork to seal., Brush the top of each pie with the remaining beaten egg, and sprinkle with sparkling sugar. Transfer the pies to the prepared baking sheet. Note: If at any time during this process the pies become sticky and hard to work with, simply refrigerate them for about 20 minutes, until firm. , Bake the pies for 18 to 20 minutes, until they're a light golden brown. Remove them from the oven, and let cool for 20 minutes before serving., Store pies, lightly wrapped, at room temperature for a couple of days; freeze for longer storage.