Unfold piecrust; fit into 9-inch pie pan. Prebake according to package directions. Remove from oven. Reduce temperature to 350 degrees F.
Mix 1-1/2 cups sugar, the flour and cocoa in medium-size heavy-bottomed saucepan. Stir in just enough milk to make paste; stir in remaining milk. Cook over medium heat, stirring, 10 to 12 minutes or until thickened. Remove from heat.
Beat yolks in medium-size bowl until thick and yellow. Carefully whisk a little of hot cocoa mixture into yolks; whisk yolk mixture back into cocoa mixture. Bring to boiling; remove from heat. Stir in vanilla and butter just until butter is melted. Pour into pie shell.
Beat egg whites in bowl just until frothy. Add cream of tartar and beat until peaks form. Beat in the 6 tablespoons sugar, 2 tablespoons at a time, until stiff peaks form. Spread meringue over pie filling so it touches all around the edge, sealing the top.
Bake in 350 degree F oven for 12 to 15 minutes or until meringue is lightly browned. Transfer to wire rack to cool completely. Makes 8 servings.