In a medium saucepan combine the water and sweet potato. Bring to boiling; reduce heat. Cover and simmer about 12 minutes or until sweet potato is very tender. Do not drain. With a potato masher, mash sweet potato in the water. Measure the sweet potato mixture. If necessary, add additional water to make 1-3/4 cups total mixture. Return sweet potato mixture to saucepan. Stir in buttermilk, the 1/4 cup sugar, butter, and salt. (Mixture may appear curdled.) Heat or cool and stir as necessary until warm (120 degrees F to 130 degrees F); set aside.

In a large mixing bowl combine 2 cups of the flour and the yeast. Add the sweet potato mixture. Beat with an electric mixer on low to medium speed for 30 seconds, scraping the side of the bowl. Then beat on high speed for 3 minutes. Using a wooden spoon, stir in as much of the remaining flour as you can.

Turn the dough out onto a lightly floured surface. Knead in enough of the remaining flour to make a moderately stiff dough that is smooth and elastic (6 to 8 minutes total). Shape dough into a ball. Place dough in a lightly greased bowl, turning once to grease the surface of the dough.

Cover and let rise in a warm place until double (45 to 60 minutes). (When poked with two fingers, dough will dent in and dents will not spring back.)

Punch dough down. Turn dough out onto a lightly floured surface. Divide dough in half. Cover and let rest for 10 minutes. Lightly grease two 8x4x2- or two 9x5x3-inch loaf pans; set aside.

On a well-floured surface roll out one portion of dough into a 15x10-inch rectangle. Combine the 1/2 cup sugar, the walnuts, and pumpkin pie spice. Brush half the dough (about 10x7-1/2 inches) with water. Using about one-fourth of the sugar mixture, sprinkle mixture over the surface brushed with water. Fold the unbrushed portion of dough over the sugar mixture, making a 10x7-1/2-inch rectangle. Roll out to a 12x8-inch rectangle. Brush surface lightly with water. Sprinkle with another one-fourth of sugar mixture to about 1 inch from edges.

Cover and let rise in a warm place until nearly double (about 30 minutes). Bake in a 375 degree F oven about 35 minutes or until bread sounds hollow when tapped. (If necessary, cover loosely with foil the last 15 minutes of baking to prevent overbrowning.) Remove bread from pans; cool completely on wire racks. If desired, drizzle loaves with Powdered Sugar Icing. Cover and store at room temperature for up to 3 days. Makes 2 loaves (about 32 servings).

If you don't have buttermilk on hand, substitute sour milk in the same amount. For each cup of sour milk needed, place 1 tablespoon lemon juice or vinegar in a glass measuring cup. Add enough milk to make 1 cup total liquid; stir let the mixture stand for 5 mintues before using it in a recipe.

In a small mixing bowl combine 1 cup sifted powdered sugar, 1 tablespoon milk, and 1/4 teaspoon vanilla. Stir in additional milk, 1 teaspoon at a time, until icing is of drizzling consistency.Make-Ahead Tip: Up to 1 month ahead, prepare and bake bread as directed; cool completely but do not drizzle with icing. Place loaves in a freezer container or bag and freeze. To serve, thaw at room temperature for 2 hours. Drizzle with icing.