Preheat oven to 425 degrees F. Line a large baking sheet with parchment paper.

Whisk together flour, salt, and baking powder. Using your hands or a processor, mix in the butter. You want the butter to be in large, pea-sized pieces distributed through the flour.

Stir in the sour cream until the sour cream is incorporated into the flour mixture and until the flour mixture begins to come together. I found this was a pretty crumbly mixture at this point; it didn't come together into a solid ball until I kneaded it a few times on my counter.

Turn the dough out onto your counter (lightly floured) , knead it a few times to form it into a ball and then shape it into a rough log. Next, using a rolling pin, roll the dough out into an 8 inch by 10 inch rectangle. Dust both sides of the rectangle with flour and, starting with the shorter end, fold the dough into three parts like a business letter.

Turn your dough over and give it a 90 degree turn. Roll it out again into an 8 inch by 10 inch rectange. Again, fold it into three.

Wrap the dough in plastic wrap and chill in your refrigerator for at least 30 minutes.

Wash and drain your berries if you are using fresh berries. Put the berries into a saucepan. Whisk cornstarch and sugar together and pour over the berries. Add salt and lemon juice and stir. Heat this mixture over medium-high heat until the liquid in the pan begins to simmer. Then turn the heat down to medium and cook for about 5 minutes. Be sure to stir the mixture as it is cooking and take it off the heat when it begins to thicken.

Remove the dough from the refrigerator and roll into a 14 inch by 14 inch square. Cut the dough into 3 1/2 inch squares. You will have sixteen squares. If the dough begins to heat up and becomes sticky and hard to handle, put it back in the refrigerator for 20 minutes or so. I've had to do this and the dough firmed up nicely.

Place one heaping tablespoonful of the blueberry filling on each of 8 squares. Brush the edges of the squares with beaten egg; this will help seal the edges of the pies.

Cut vent holes in each of the remaining squares. Place these dough squares over the squares with the blueberry filling. Press the edges closed using the tines of a fork.

Brush the top of each hand pie with beaten egg and sprinkle with sparkling sugar. Again, if the dough has become soft and sticky, put the hand pies into the refrigerator for about 20 minutes to firm up.

Bake for 18-20 minutes at 425 degrees F. When they are done, your hand pies will be a light golden brown color.

Remove the hand pies from the oven and let them cool on your counter for about 20 minutes.