Make the dough: In a bowl stir together the flour, the sugar, and the salt. Add the butter, and blend the mixture until it resembles coarse meal. Add the yolk mixture, toss the mixture until the liquid is incorporated, and form the dough into a ball. Dust the dough with flour and chill it, wrapped in plastic wrap, for 1 hour.
Make the filling: In a large bowl stir together the cornstarch, the rum, the lemon juice, the sugar, and the cinnamon. Add the blueberries, and combine the mixture well.
Pour the filling into a 6- to 7-cup gratin dish or other shallow baking dish. Roll out the dough slightly larger than the dish. Roll out the dough slightly larger than the dish on a floured surface and drape it over the filling. Fold the overhang under, pressing the dough to the edge of the dish, and crimp the edge decoratively. Make slits and holes in the crust for air vents, brush the crust with the egg wash, and sprinkle it with the sugar. Bake the pie on a baking sheet in the middle of a preheated 375 degree F oven for 1 hour to 1 1/4 hours, or until the filling is bubbly and the crust is golden. Transfer the pie to a rack and let it cool. The pie may be made 1 day in advance and kept at room temperature.
Recipe courtesy of Gourmet Magazine