Journalist Nancy Grace joins Martha for Cooking School Lesson No. 18: blueberry pie and pie crust 101.
On a lightly floured piece of parchment paper, roll out one disk of dough to a 12-inch round. With a dry pastry brush, sweep off excess flour; fit dough into a 9-inch glass pie plate, pressing it into edges. Trim dough to a 1/2-inch overhang all around. Fold edge of dough over or under, and crimp as desired. Roll out remaining dough in the same manner; transfer dough (on parchment) to a baking sheet. Chill pie shell and dough until firm, about 30 minutes.
Place blueberries in a large bowl; with your hands, crush about 1/2 cup of berries, letting them fall into the bowl as you work. Add sugar, cornstarch, and lemon juice; stir to combine. Spoon mixture into chilled pie shell, mounding berries slightly in the center. Dot with butter. Remove dough from refrigerator, and place over blueberry filling. Tuck edge of top dough between edge of bottom dough and rim of pan. Using your fingers, gently press both layers of dough along the edge to seal, and crimp as desired.
Using a paring knife, cut several vents in top of dough to allow steam to escape. In a small bowl, whisk together egg yolk and cream. Brush surface with egg wash, being careful not to let it pool. Freeze or refrigerate pie until firm, about 30 minutes. Meanwhile, preheat oven to 400 degrees, with rack in lower third.
Place pie on a parchment-lined baking sheet. Bake until crust begins to turn golden, about 20 minutes. Reduce oven temperature to 350 degrees. Continue baking, rotating sheet halfway through, until crust is deep golden brown and juices are bubbling and have thickened, 40 to 50 minutes more. Transfer pie to a wire rack to cool completely. The pie is best eaten the day it is baked, but it can be kept at room temperature, loosely covered with plastic wrap, for up to 2 days.
To make an extra-wide lattice-top crust, roll out the dough to 1/8 inch thick, and cut four 3-inch-wide strips. Chill the strips until firm. Weave the strips over the filling, and seal the lattice to the side of the bottom crust with a wash of egg and milk.
On the Martha Stewart Show, Martha substituted 1/4 cup cornstarch with 2 tablespoons cornstarch and 2 tablespoons flour.