Scrub potatoes, slice and boil in water to cover until soft. Place the potatoes and cooking water in blender container and add water to bring measure to 3 cups; puree and transfer to large bowl. Stir in brown sugar and cool to lukewarm. Sprinkle yeast over, stir to dissolve and let rest at room temperature 45 minutes. Stir in 1 1/2 cups of the flour and beat vigorously about 3 minutes until air bubbles form. Stir in butter and salt, beat well and gradually add 1 1/2 cups of the remaining flour. Let rest at room temperature 45 minutes. Mound remaining flour on board, turn dough out and, using a steel baker's scraper, knead in the flour until dough is stiff enough to knead by hand. Adding additional flour as needed to prevent sticking, knead 3 to 5 minutes. Dough willl be soft and slightly sticky. Divide into 2 equal portions, knead briefly, form into 2 loaves and place in 2 oiled loaf pans. Cover with tea towel and let rise in a warm place 45 minutes or until almost double in size. Place in oven, turn heat to 350 degrees F and bake 45 minutes or until bread tests done. Turn out onto wire rack, turn right side up and cool.