Preheat oven to 375° F.
In a mixing bowl, combine the mashed potatoes, sugar, yeast, and
lukewarm water. Cover with plastic wrap and let rise 2 hours. Into the mixing bowl, mix the milk, eggs, sugar, butter,
shortening, salt, and flour.
Cover with plastic wrap and let rise
until double in size. Flour board and lightly knead dough. Roll out to 1/4-inch
thickness. Brush with melted butter and cut into 2-inch squares.
Bring up the corners of each square to the center like an
old-fashioned valentine. Place on greased cookie sheet, well apart.
Cover and allow rising for 30 minutes. Place in oven and bake 15 to 20 minutes. When you remove from
oven, dust with powdered sugar.