1. Dissolve yeast in warm water along with 1 tablespoon sugar; allow to stand for five minutes.
2. Cream butter and remaining sugar until light.
3. Add milk to mashed potatoes and combine with creamed butter.
4. Cool to lukewarm and add to yeast mixture with egg and salt.
5. Beat in 2 cups of the flour and set aside to rise until light.
6. Knead in remaining flour; return to bowl, brush top with shortening and cover closely.
7. Store in refrigerator until ready to use.
8. Shape as desired; let stand in a warm place until doubled.
9. Bake at 400F for 15 to 20 minutes.