On a lightly floured surface, roll dough to a 1/8-in.-thick circle; transfer to a 9-in. pie plate. Trim crust to 1/2 in. beyond rim of plate; flute edge. Refrigerate 30 minutes. Preheat oven to 425°. Line crust with a double thickness of foil. Fill with pie weights, dried beans or uncooked rice. Bake on a lower oven rack until edge is golden brown, 20-25 minutes. Remove foil and weights; bake until bottom is golden brown, 3-6 minutes longer. Cool on a wire rack.
In a large saucepan, combine sugar, cornstarch and salt. Stir in milk and chocolate. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes longer. Remove from the heat.
Stir a small amount of hot filling into egg yolks; return all to the pan, stirring constantly. Bring to a gentle boil; cook and stir 2 minutes longer. Remove from the heat.
Gently stir in butter and vanilla until butter is melted. Spoon into crust. Cool on a wire rack. Cover and chill for at least 3 hours. If desired, serve with whipped cream and grated chocolate.
Let pie weights cool before storing. Beans and rice may be reused for pie weights, but not for cooking.
Editor's Note Combine 1-1/4 cups all-purpose flour and 1/4 tsp. salt; cut in 1/2 cup cold butter until crumbly. Gradually add 3-5 Tbsp. ice water, tossing with a fork until dough holds together when pressed. Shape into a disk; wrap and refrigerate 1 hour.