Combine flours, sugar and salt in the bowl of a food processor. Add in cold butter and cream cheese and pulse until the butter is the size of peas. Drizzle in apple cider vinegar, followed by the water, one tablespoon at a time, until the dough just comes together.
Tip the dough out onto a floured work surface and divide into two equal pieces. Gently shape each piece into a square, wrap in plastic wrap and chill for at least 30 minutes in the refrigerator.
While the dough is chilling, drain and reserve the juice from the blueberries. Combine the drained blueberries, flour, cornstarch, sugar, ¼ cup blueberry juice and lemon juice in a small saucepan. Heat on medium, stirring occasionally for 8-10 minutes or until thickened. Set aside to cool.
Flour your work surface and rolling pin. Working with one piece of dough at a time, roll the chilled dough into an 18” x 8” rectangle. With a sharp knife, cut the dough into twelve 3” x 4” pieces.
To assemble, brush the edges of one pastry piece with egg wash, add 1 ½ tbsp of blueberry filling and top with another piece of dough. Lightly press the edges together then seal, crimping all edges with the tines of a fork. Cut a small ½” slit to allow for venting. Repeat with remaining dough, creating 12 hand pies total. Freeze assembled pies for 30 minutes or until solid.
Preheat the oven to 400ºF.
Brush the hand pies with remaining egg wash and bake from frozen on a parchment lined tray, about 26-28 minutes or until golden brown. Let cool completely.
Soften the butter and cream cheese in the microwave for 15-30 seconds until very soft, but not hot. Whisk in powdered sugar and blueberry juice to make the frosting. Drizzle the frosting on top of the cooled hand pies and top with sprinkles, if desired.