Sour Cream Coffee Cake w/ Walnuts

I remember when I first saw this recipe and it changed everything for me when it came to cooking sugar-free. So many things that I craved that I couldn't have. So I made this to fit my personal tastes, but if you want the original recipe without my changes, go to Joy Bauer's website.

INGREDIENTS FOR TOPPING
2/3 cup flour, all-purpose
1/3 cup sugar substitute
1 tsp ground cinnamon
1/2 tsp ground nutmeg
1/4 cup margarine, trans-fat free, reduced-fat, soft tub, frozen for 15 minutes

INGREDIENTS FOR CAKE
3/4 cup flour, all-purpose
3/4 cup flour, whole-wheat
1 1/4 cup sugar substitute
1 1/2 tsp baking powder
1 tsp baking soda
1/4 tsp salt
1 cup sour cream, nonfat
2 eggs
1/4 cup oil, canola
2 tsps vanilla extract
1 tsp maple extract
1/2 cup nuts, toasted walnuts, coarsely chopped

DIRECTIONS

1. Preheat the oven to 350°F. Grease a 9" round springform pan. Line the bottom with a round of parchment paper or coat with cooking spray.
2. To make the topping, in a small bowl, stir together the flour, sugar substitute, cinnamon, and nutmeg. Add the margarine and using a pastry cutter or your fingers quickly cut or rub in the margarine until coarse crumbs form. Cover and refrigerate.
3. To make the cake, on a piece of wax paper or aluminum foil, sift the all-purpose flour, whole wheat flour, sugar substitute, baking powder, baking soda, and salt. In a large bowl, whisk together the sour cream, eggs, oil, vanilla, and maple extract until well blended. Add the flour mixture. Using an electric mixer on medium speed or a wire whisk, beat until smooth and creamy, 1 to 2 minutes.
4. Spoon half of the batter into the prepared pan and spread evenly. Sprinkle evenly with half of the topping. Cover evenly with the remaining batter. Sprinkle evenly with the nuts, gently pressing them into the batter. Cover with the remaining topping.
5. Bake until the topping is golden brown and a toothpick inserted into the center of the cake comes out clean, 40 to 45 minutes. Transfer the pan to a wire rack and let cool for 20 minutes. Remove the sides of the springform pan. Serve the cake warm or at room temperature, cut into wedges.