Dissolve yeast and sugar into 1/4 cup of milk. Add to 1 pint milk that has been scalded, mashed potatoes, shortening, sugar, and flour. Mix well and let rise 2 hours. Mix in baking soda, baking powder, and salt. Add enough flour to make soft dough, let rise 1 hour. Roll out the Denver biscuits on floured board, place in greased pan. Let rise 2 hours. Bake your Denver biscuits at 400 degrees for 20 minutes.