1. Grease two 8x8 square pans (or two 8 rounds, or one 9x13).
2. Mix flour, baking powder, baking soda& salt together; set aside.
3. Cream margarine and the 1cup sugar until light & fluffy.
4. Add eggs& vanilla; beat thoroughly.
5. Blend in sour cream alternately with flour mixture; stir in nuts.
6. Divide& spread batter evenly in pan (s).
7. Combine topping ingredients& sprinkle on top of batter.
8. Bake at 350 for 35-40 minutes (40-45 if using 9x13).
9. Notes: Topping is very crumbly; it can be used as streusel in middle of cake if desired.
10. Also, this cake freezes well!