Combine flour, salt, baking powder, and 2 tablespoons sugar in a large bowl. Add the chilled cubed butter and use a pastry cutter or your fingertips to work it into the flour until the mixture resembles coarse meal with pea-sized butter pieces throughout—this is crucial for creating flaky layers. Stir in the sour cream until just combined, then knead gently on a lightly floured surface until the dough comes together. Shape the dough into a 10-inch log, then roll it into a 10x12-inch rectangle. Fold the dough like a letter (fold one long end to the middle, then fold the other end over top), then roll it out again into a 10x12-inch rectangle and fold once more. I like to perform this lamination step because it creates those beautiful, delicate layers that make hand pies special. Wrap the folded dough tightly in plastic wrap and refrigerate for at least 1 hour—this resting period allows the gluten to relax and the butter to solidify, ensuring the best flaky texture.
While the dough chills, combine the fresh blueberries, 1/3 cup sugar, cornstarch mixture, lemon juice, lemon zest, and vanilla extract in a saucepan over medium heat. Simmer for about 6 minutes, stirring occasionally, until the mixture thickens and the blueberries break down slightly—you should see the filling coat the back of a spoon. Transfer the filling to a shallow bowl or plate and let it cool to room temperature. This cooling step is important because a warm filling will soften the dough and cause the hand pies to fall apart during baking.
Preheat your oven to 425°F and line a large baking sheet with parchment paper. Having everything ready before you start assembling ensures the hand pies don't sit around waiting to bake, which can make them soggy or cause the filling to leak out during cooking.
Remove the chilled laminated dough from the refrigerator and place it on a lightly floured surface. Roll it out to a 16x16-inch square, then cut it into 16 equal pieces (you can cut a 4x4 grid). This will give you 8 bottom pieces and 8 top pieces for your hand pies. Place 2 tablespoons of the cooled blueberry filling from Step 2 onto the center of 8 dough pieces, leaving about a 1/2-inch border around the edges. Lightly brush the exposed dough borders with the beaten egg using a pastry brush. Take the remaining 8 dough pieces and cut 2-3 small slits or vents in each one to allow steam to escape during baking, then place them on top of the filled pieces.
Press the edges of each hand pie firmly with the tines of a fork, sealing the two layers together while creating a decorative pattern. Brush the top of each pie generously with the remaining beaten egg wash, then sprinkle the turbinado sugar over the tops—this creates a beautiful, sparkly crust. Place the assembled pies on the prepared baking sheet, spacing them about 2 inches apart to allow for heat circulation.
Bake the hand pies in the preheated 425°F oven for about 20 minutes, until the pastry is deep golden brown and the filling is bubbling slightly at the vents. Remove from the oven and let the hand pies cool on the baking sheet for at least 20 minutes before serving—they'll continue to set and firm up during this time, making them easier to handle and more enjoyable to eat.
