In a small saucepan, heat milk until bubbles form around the edge of the pan; remove from heat. Add quarter cup sugar, then salt and half cup butter; stir until butter melts; add mashed potato; cool to lukewarm. If possible, check temperature with thermometer. Sprinkle yeast over water in large mixer bowl; stir to dissolve. Add milk mixture and 3.5 cups of flour; beat with electric mixer until smooth--2 minutes. Stir in raisins. Gradually add remaining flour; mix in last by hand until dough is stiff and leaves side of bowl. Turn dough onto lightly floured pastry cloth or board. Knead until smooth and elastic-10 minutes. Place in greased bowl; turn dough to bring up greased side. Cover with towel; Let rise in warm place (85F), free from drafts, until doubled-- 1.5 hours. Mix half cup sugar and cinnamon. Turn dough on lightly floured pastry cloth. Roll one half into 16x8 inch rectangle. Sprinkle with half of cinnamon-sugar. From narrow side, roll up, jelly roll fashion. Pinch edges and ends together. Tuck ends under. Place, seam side down, in greased 9x5x3 inch loaf pan. Brush with 1 tablespoon butter. Cover with towel. Repeat with other half of dough. Let rise in warm place, free from drafts, until sides come to top of pans and tops are rounded--1 hour. Place oven rack in middle of oven. Preheat oven to 190ºC . Brush each loaf with rest of butter. Bake 35 to 40 minutes--tops should be well browned. Remove from pan at once; cool slightly on rack. away from drafts. Sprinkle with sugar. Serve warm. Makes 2 loaves.