On a lightly floured surface, roll dough to a 1/8-in.-thick circle; transfer to a 9-in. pie plate. Trim 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 completely on a wire rack.
In a large saucepan, combine the first 6 ingredients. Cook and stir over medium-high heat until thickened and bubbly, about 2 minutes. Reduce heat; cook and stir 2 minutes longer. Remove from the heat. Whisk 1 cup hot mixture into egg yolks. Return all to the pan; bring to a gentle boil, stirring constantly.
Remove from the heat; stir in butter and vanilla. Cool slightly. Pour warm filling into crust. Cool for 1 hour. Chill until set. If desired, top with whipped cream and sprinkle with cocoa to serve.
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Dough for single-crust pie
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.