In a large bowl, cream butter; add sugar, pumpkin, milk and egg. Sift dry ingredients over. Beat until blended. Stir in orange rind. Pour into greased and floured 1-pound coffee can. Cover with foil and secure. Place on rack in crockpot. Pour 2 cups hot water into cooker. Cover crockpot and cook on High for 3 to 4 hours. Check water level at least once. Turn out on rack to cool.