Combine all the filling ingredients in a medium saucepan and bring it to mixture to a boil on medium–high heat.
Cook the filling for about 8 minutes (it will thicken at around 5 minutes then loosen up again, but don’t worry it will set/thicken overnight in the fridge), stirring often to make sure it doesn’t catch and burn on the bottom.
Transfer the filling to a heat-proof bowl, cover with plastic wrap pressing it down so that it touches the filling surface. Chill overnight.
To make the pie crust dough
In a giant bowl (yes, the GIANTer, the better!), whisk together all the dry ingredients of the filling.
Scatter the butter chunks over top, then with your hands, toss them so they are all coated with flour. Now, turn the chunky mixture to sand by working it quickly. Basically pick up and rub (a.k.a. “sand) it between your palms. Do not use force. Be gentle. You can also use your finger tips if you prefer, just don’t let the butter get too warm.
Make a well in the center and add the water, working it into the mixture gradually with your finger tips or a fork, then quickly working it in with your hands.
As soon as the dough is mostly moistened and clumps when squeezed, stop. Press it into two equal disks (there may be dry bits. That’s okay), wrap with plastic wrap and chill overnight.
To assemble the blueberry hand pies
Preheat the oven to 400 °F (200 °C) with a baking sheet placed on the middle rack, upside down. Take the dough out of the fridge about 20 minutes before you roll it.
Roll half the dough out to about ⅛ inch thickness on a lightly floured surface. Using a 4″ round cookie cutter, cut out as many rounds as you can. Re-roll the scraps and cut out more.
Transfer the rounds on two parchment lined baking sheets (you should have 9 or 10).
Brush a ½ inch border around the edge of each round with the egg and place about 1 tablespoon of chilled filling in the middle of each. These are your “pie bottoms”.
Roll out the rest of the dough and cut out 9 or 10 more rounds. Brush a ½ inch egg wash border on each and flip each onto a pie bottom. Press the edges together with your finger tip and then use fork tongs to press/mark the edge even more.
Brush the assembled hand pies with egg wash and sprinkle with turbinado. Cut slits into the top.
Bake the pies (1 sheet at a time), setting the baking sheet directly on the one that is preheated in the oven for 20 to 25 minutes til the tops are evenly golden brown.
Let cool slightly (or completely) before serving.
Notes
I used European-style Churn 84 unsalted butter from Stirling Creamery for this recipe.