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.

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.

Preheat the oven to 425°F; place a rack on the middle shelf. Line a baking sheet with parchment paper.

Roll the dough into a 14" x 14" square. With a straight edge and pastry wheel, or a 3 ½" square cutter, cut out sixteen 3 ½" 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.

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.