Dissolve yeast and 1/2 teaspoon sugar in warm water; let stand 5 minutes or until bubbly.
Combine potatoes, 1 cup sugar, shortening, 1/4 cup butter, and salt in a large mixing bowl; stir until shortening melts. Stir in yeast mixture.
Cover and let rise in a warm place (85°), free from drafts, 1 1/2 hours or until spongy.
Stir in eggs and flour to make a soft dough. Shape dough into a ball. Place in a greased bowl, turning to grease top. Cover and repeat rising procedure 2 hours or until doubled in bulk.
Turn dough out onto a lightly floured surface; knead 5 minutes or until smooth and elastic. Divide dough in half; pat evenly into 2 greased 13- x 9- x 2-inch baking pans. Cover and repeat rising procedure 2 hours or until doubled in bulk.
Press slices of butter into dough in rows at 1/2-inch intervals. Top each slice of butter with 1 teaspoon brown sugar. Sprinkle 1 teaspoon cinnamon over each cake. Bake at 375° for 20 minutes or until golden brown.