Preheat the oven to 375°F. Line a baking sheet with parchment paper.

Place the huckleberries, sugar, and cornstarch in a mixing bowl and stir to combine. Add the lemon juice and let sit for 5 minutes to macerate.

Unwrap the pastry dough and lay the two sheets out on the prepared baking sheet. Cut each piece of pastry into four squares and place a heaping tablespoon or two of huckleberries right in the centre of each square. Lightly beat the egg and water together in a small bowl to make an egg wash and brush the edges of the squares with some of it. Fold over the pastry to enclose the pie filling pressing lightly to seal. Brush the tops of the pies lightly with the remaining egg wash and sprinkle with the golden sugar.

Bake for 20–25 minutes, or until the puff pastry is golden brown. Let cool on a wire rack for at least 10 minutes to ensure the filling is cool enough to eat.

These keep in an airtight container at room temperature for up to 3 days.