Remove the pie crust mix from cardboard box and place in the freezer for about 30 minutes. Once chilled, pour contents out into a metal bowl and add the cubed cold butter. Working quickly with your hands, pinch the butter into the flour until it leaves behind a sand-like texture.
Slowly drizzle in ice water, one tbsp at a time until you can form a dough ball and the dough sticks together just enough without crumbling away.
Wrap in plastic wrap and put into the fridge for 2 hours or overnight.
When ready to make your hand pies, remove from fridge and roll out into a 1/4 thick round on a floured marble slab. Cut 6" circles out of the dough and place onto a parchment lined cookie sheet.
For the pie filling
Combine blueberries, sugar, lemon zest and cornstarch into a bowl.
To assemble the pies
Preheat oven to 400°F.
Whisk together the egg and tbsp of water to make an egg wash.
Using a pastry brush, brush the rim of the 6" round of dough with roughly 1" of egg wash on the edge.
Use a spoon to spoon about 1/4 cup of filling into the middle of the mixture.
Carefully take one edge and fold the pie in half, being careful not to rip the dough and pressing the edges firmly onto the other side. Repeat for remaining hand pies.
Using a fork, press the edges of each hand pie together to create decorative ridges, this step is optional but it does look nice. Then using a small sharp knife, slice three slits in the middle filling area of the hand pie to allow steam to vent while it cooks.
Using a pastry brush, brush the tops of the hand pies with the egg wash and sprinkle with sugar to finish.
Place hand pies in the oven and bake for 15-20 minute until the tops of the pies turn deep golden brown. Remove and allow to cool on a wire rack for minimally 30 minutes or overnight.