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 unpricked crust with a double thickness of foil. Fill with pie weights, dried beans or uncooked rice. Bake on a lower oven rack until edges are light golden brown, 15-20 minutes. Remove foil and weights; bake until bottom is golden brown, 3-6 minutes longer. Cool on a wire rack. Reduce oven setting to 350°., In a saucepan, mix sugar, cocoa, cornstarch and salt; gradually add milk. Cook and stir over medium-high heat until thickened and bubbly. Reduce heat; cook and stir 2 minutes more. Remove from heat. Stir about 1 cup of the hot filling into the egg yolks. Return to saucepan and bring to a gentle boil. Cook and stir 2 minutes. Remove from the heat and stir in vanilla. Pour hot filling into pie crust. , For meringue, immediately beat egg whites with cream of tartar until soft peaks form. Gradually add sugar and continue to beat until stiff and glossy. Spread evenly over hot filling, sealing meringue to pie crust. Bake until meringue is golden brown, 12-15 minutes. Cool 1 hour on a wire rack. Refrigerate at least 4 hours before serving.