Combine potatoes and butter; stir until butter melts. Add sugar, baking powder, salt, nutmeg, eggs, lemon rind and juice, milk, whipping cream, and vanilla; beat just until smooth.
Pour filling into pastry shell; bake at 425° for 8 minutes. Reduce heat to 350°; bake an additional 40 minutes or until a knife inserted in center comes out clean. Cool pie before serving.