Cut the cold butter into cubes, then rub it into the flour, salt, and sugar with your hands until the mixture looks like coarse meal.

Stir in the sour cream with a fork, then knead the dough briefly until it comes together and feels smooth.

Split the dough in half and flatten each piece into a disk. Wrap both disks in plastic and refrigerate while you prepare the filling.

Sort through the mulberries and remove any twigs or debris. Toss the berries with both sugars, lemon zest, lemon juice, vanilla, cinnamon, and cornstarch.

If using frozen berries, use them straight from the freezer without thawing.

Preheat the oven to 375°F and place a rack in the lower position.

Roll out one disk of dough and fit it into a 9-inch pie pan, leaving a small overhang around the edges. Pour in the mulberry filling.

Roll out the second disk and place over the filling, or cut it into strips for a lattice top. Crimp the edges well, cutting steam vents if using a solid top.

Brush the top with the beaten egg and sprinkle with coarse sugar if desired. Shield the rim with foil or crust protectors and bake for 45 to 60 minutes, turning the pie 180 degrees halfway through.

If the crust browns too fast, lower the oven to 350°F and tent the top with foil.

When done, the crust should be deep golden and the filling bubbling in the center. Let cool on a wire rack for about 20 minutes, or up to 2 hours for cleaner slices.

Serve with whipped cream or vanilla ice cream for the best finish.