2 env. active dry yeast
1/2 c. sugar
1 c. warm water (105 to 115 degrees)
1/2 c. firmly packed unseasoned mashed potatoes
2 tbsp. nonfat dry milk powder
1 tsp. salt
1/2 c. (1 stick) unsalted butter, melted
2 eggs
4 1/2 to 5 c. unsifted all purpose flour
1/2 c. (1 stick) unsalted butter, melted
3/4 c. firmly packed light brown sugar
1 tsp. ground cinnamon

Combine yeast and 1/2 teaspoon of the sugar in 1/2 cup of the warm water in large mixer bowl; stir to dissolve. Let stand 5 minutes.

Add remaining sugar, remaining 1/2 cup water, potatoes, dry milk, salt, 1/2 cup melted butter, eggs and 1 cup of the flour to yeast mixture. Beat at low speed until blended; then beat at high speed until very smooth, about 2 minutes. Beat in 3 cups of the remaining flour, 1 cup at a time.

Turn dough out onto lightly floured surface. Knead about 2 minutes, using 1/2 to 1 cup of the remaining flour to make soft dough that looks smooth. Grease two 15 x 10 x 1 inch jelly roll pans well. Divide dough in half. Place each half into prepared pan. With well buttered hands, flatten dough over bottom of pan, pushing well into corners. Brush about 2 tablespoons of the melted unsalted butter over each top. Place pans in warm place, away from drafts. Let rise, uncovered, until doubled in bulk, 25 to 35 minutes. (The butter will keep dough from drying out.)

While dough rises, combine brown sugar and cinnamon in small bowl. With fingers, poke deep holes over surface of both pans of dough. Sprinkle as evenly as possible with cinnamon-sugar. Drizzle with remaining melted butter. Let rise in warm place, away from drafts, until dough approaches rim of pans, 10 to 15 minutes.

Bake in preheated moderate oven (375 degrees) for 18 to 20 minutes until tops are browned and cakes feel spongy firm. Cool in pans on rack 15 to 20 minutes. Cut into large squares to serve. Makes 2 large flat cakes (12 servings each).