Line a 9-in. pie plate with pastry; trim and flute edges. Line pastry shell with a double thickness of heavy-duty foil. Bake at 450° for 8 minutes. Remove foil; bake 5 minutes longer. Cool on a wire rack.
In a small saucepan, combine eggs and sugar until blended. Cook over low heat, stirring constantly, until mixture reaches 160° and coats the back of a metal spoon. Remove from the heat; stir in vanilla. Cool to lukewarm (about 90°), stirring occasionally.
In a large bowl, beat butter until light and fluffy. Beat in cocoa until blended. Add the cooled sugar mixture; beat on high speed for 5 minutes. Spread evenly into crust. Refrigerate for 4-6 hours before serving. Refrigerate leftovers.