Combine first 4 ingredients in a heavy saucepan. Mix well to remove lumps. Gradually add milk, stirring until blended. Cook over medium heat, stirring constantly, until mixture thickens and comes to a boil; boil 1 minute, stirring constantly. Remove from heat.
Beat egg yolks until thick and lemon colored. Stir one-fourth of hot mixture into yolks; add to remaining hot mixture, stirring constantly. Cook over medium heat 2 minutes, stirring constantly. Remove from heat; stir in butter and vanilla. Pour into pastry shell.
Combine egg whites (at room temperature) and cream of tartar; beat until foamy. Gradually add remaining sugar, 1 tablespoon at a time, beating until stiff peaks form. Spread over filling, sealing to edge of pastry. Bake at 400° for 8 minutes or until lightly browned. Cool.