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 1.5 cups of the flour and beat vigorously about 3 minutes until air bubbles form. Stir in butter and salt, beat well and gradually add 1.5 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 180ºC and bake 45 minutes or until bread tests done. Turn out onto wire rack, turn right side up and cool.