Mix 2 cups flour, sugar, salt and undissolved yeast in a large mixing bowl.
Combine sour cream, water and butter; heat to between 120° to 130°F. (Note: butter may not melt completely). Gradually add to dry ingredients and beat for 2 minutes at medium speed of electric mixer. Add eggs and 1 cup flour. Stir in enough remaining flour to make a soft dough. Turn out onto lightly floured surface; knead 8 to 10 minutes to form a ball. Cover and let dough rest 10 minutes.
For filling, melt butter over low heat. Stir in sugar and maple flavoring. Add walnuts; blend well. Set aside.
Divide dough into 4 equal pieces. Roll out each piece into a 14 x 12-inch rectangle. Spread each with an equal amount of nut filling. Roll each up from the long side; seal edges. Place on greased baking sheets; sealed edges down. Cover. Let rise in a warm, draft free place until doubled in bulk, about 1 hour.
Bake rolls at 350°F for 35 minutes, or until done. Remove from baking sheets and cool on wire racks.
For frosting, combine sugar, milk and vanilla until mixture reaches desired consistency. Drizzle over each roll.
Variation: To make Pecan Nut Rolls, use the following filling. Melt 1 cup butter over low heat. Stir in 1/2 cup sugar and 3 tablespoons vanilla extract. Add 7 cups ground pecans; blend well.