Squeeze each grape so the pulp slips into a saucepan and reserve all of the purple skins in a bowl.
Cook the pulp for 8 minutes until it releases its seeds, press it through a coarse sieve, and discard the seeds.
Combine the strained pulp with the reserved skins, sugar, cornstarch, lemon juice and salt and cool for 15 minutes while heating the oven to 400°F.
Fit one rolled dough disk into a 9-inch plate, add the grape filling, dot with butter, and cover with the second rolled crust.
Trim and crimp, cut vents, brush with egg wash, and bake for 20 minutes before lowering the oven to 375°F.
Bake 30 to 35 minutes more until the center bubbles, then cool for 4 hours before slicing.