Pare potatoes and cook in boiling water until tender.
Drain (reserve 1 cup of the water) and mash potatoes until smooth.
Sprinkle yeast over warm water to dissolve.
Combine hot mashed potatoes with butter, shortening, sugar, and salt.
Mix thoroughly and, when cooled to lukewarm, stir in 1 cup of potato water and the dissolved yeast.
Cover with a tea towel and place in a warm spot until spongy looking.
Beat the mixture with eggs and enough flour to make a soft dough.
Cover again and let rise in a warm place until double in size.
Divide dough in half and make an even layer in 2 greased 11 1/4 X 7 1/2 X 1 1/2 inch baking pans.
Let rise the third time until light, then with your finger poke holes in the dough, filling the holes with generous amounts of butter and brown sugar.
Sprinkle top with cinnamon and bake in a preheated 375° oven for 20 to 30 minutes.
Cool and cut into squares to serve.