Heat water to 105 - 100ºF. Put in a large bowl. Sprinkle yeast over water, stir until dissolved. Add sugar, salt, softened margarine, eggs and 3 cups of flour. Beat until smooth, about 4 minutes. Add remaining flour and again beat until smooth. Cover and let rise in a warm place until doubles, about 2 hours. Place in refrigerator overnight. For filling stir together margarine, brown sugar and yolks. Stir in milk and vanilla. Fold in  nuts. Stir down dough. Turn out on floured surface. Work with half of dough keeping other half in refrigerator. Roll first half into a 14 x9 inch rectangle. Brush with melted margarine. Spread half of filling over dough. Roll from both sides to center making a 9 inch loaf. Turn over smooth side up and place in greased pan. Brush top with melted margarine. Repeat for second loaf. Cover, let rise for 1 1/2 hours or until doubles in size. Bake at 350ºF. for 35 minutes. Remove from pan. Sprinkle with powdered sugar.