Scald milk, add shortening, sugar, and mashed potatoes. Cool to lukewarm. Mix yeast and warm water. Let stand 10 minutes. Add milk, shortening, sugar, and mashed potatoes. Stir. Stir in eggs and vanilla. Sift togrther the flour, baking powder, salt, and add to other ingredients and mix. Add 1/2 cup more flour if needed. Dough should be soft and sticky. Put in greased bowl and turn to coat. Let rise for 1 1/2 hours. Roll out on floured board to 1/2 inch thickness. Cut with donut cutter. Let rise 1/2 hour or more. Drop in hot grease until golden brown. Glaze with powdered sugar mixed with boiling water and vanilla. Dip right after frying and let dry.