In a food processor, mix flour and salt.

Add very cold butter to flour mixture all at once and pulse until mixture resembles a coarse meal, with just a few larger pieces remaining.

Mix egg yolk with sour cream and add to coarse mixture and pulse until you the dough will hold together when squeezed. If it's too dry, add sour cream to the mixture, tablespoon by tablespoon.

Divide dough into 4 equal pieces (approximately 3 ounces each), form into disks and wrap in parchment or wax paper.

Refrigerate the dough for at least one hour, or up to 2 days in the refrigerator or one month in the freezer.

Mix blueberries sugar, cornstarch, or Clear Jel and cinnamon together. Mash a few of the blueberries to release their juice. You don't want it soupy, but just enough to incorporate the sugar and starch so that those ingredients are no longer dry.

Set aside.

At the end of 1 hour, roll each dough disk to a 1/8 inch thickness, which will form a circle of approximately 6-6 1/2 inches.

If you are using a hand pie press, flour your press and lay your flattened dough disk on the press.

Brush the entire disk, the side where your filling will be, with egg whites.

Place 1/4 cup of blueberry mix in the middle of the disk and fold over, crimping down the edges.

Make 3 small (1/2 inch) cuts in the crust and place on a parchment-lined tray that will fit in your freezer and complete the process for the other 3 dough disks.

Note: the dough is difficult to work with when it warms up, so work quickly or keep them in the fridge until you work with them.

Once all 4 are done, place them in the freezer for at least one hour. You can make them ahead of time and keep in an airtight container and just bring out to bake as needed.

Preheat oven to 400'F.

When the oven is heated, remove hand pies from the freezer and brush with egg yolk. Sprinkle with sugar if desired.

Place in oven and set a timer for 10 minutes.

At the end of 10 minutes, reduce temperature to 350'F and bake for 10-15 longer.

Enjoy the deliciousness!