In a large bowl, combine the dry ingredients: all-purpose flour, buckwheat flour, sugar, salt and cardamom. Using a box grater, grate the cold butter atop the flour mixture. Working quickly, and using your hands, break the butter bits into the flour until they’re evenly distributed and resemble the size of small peas. Add 1/2 cup of water; mix the dough together until it just begins to clump together. The dough will shaggy. Add one tablespoon at a time until it starts to come together. I added an extra 4 tablespoons (1/4 cup) of water.
Remove the dough from the bowl and place it on a lightly floured counter. Knead the dough a few times until it comes together and shape it into a mound. Shape the disc into 2 discs and wrap them in plastic wrap; transfer them to the refrigerator to chill for 1 hour or overnight.
While the dough is chilling, make the filling! To a medium saucepan, add the blackberries, tapioca starch, sugar, salt, cardamom and nutmeg. Over medium heat cook the mixture for 5 minutes, until the fruit has softened.
Remove one of the discs of dough from the refrigerator. Liberally flour your work surface and rolling pin. Begin to roll the dough, being sure to rotate it every so often to avoid sticking, to a 14-inch round. Using a 3-inch biscuit cutter, cut 6 circles out. Repeat the process with the second disc of dough. You should end up with about 12 circles of dough.
Spoon a small amount of the blackberry mixture into the center of the dough, leaving about 3/4-inch space around the edges. Note: you might have to halve some of the blackberries since the larger ones will be too big. Brush egg wash along the edges of each of the dough circles. Fold the dough over the filling, pressing the edges to seal. Using the tines of a fork, sealing the edges. Repeat the process until all of the hand pies are made. Transfer the pies to a parchment-lined baking sheet and place in the hand pies in the freezer for 10 minutes.
Meanwhile, preheat the oven to 400 degrees F. Brush the tops of the hand pies with egg wash and sprinkle with turbinado sugar. Bake the pies for 15-17 minutes, until the tops and edges are golden brown. Remove from the oven and transfer to a cooling rack. Eat warm or room temperature.