Mix flour, baking powder, baking soda, cinnamon and salt and set aside.
Cream margarine and sugars until light and fluffy. Add eggs, one at a time,
beating well. Stir in buttermilk and then add dry ingredients.
Spread batter into greased and floured 9"x13" pan.
Mix all ingredients for topping and sprinkle over batter.
Refrigerate overnight or for at least 8 hours.
Bake at 350 degrees for 45-50 minutes.
Variations:
Substitute coconut for the nuts in the topping, or use a combination of nuts and coconut.
Pour batter into 2 round cake pans instead of the 9"x13" pan. I often do this.
We eat one of the coffeecakes, and I freeze the other one for later use.