1. Heat oven to 350 degrees F. Coat 9-inch square baking pan with cooking spray.Topping:
2. In medium-size bowl, stir together light brown sugar, flour, cinnamon and salt. Cut butter into mixture and refrigerate.Cake:
3. In large bowl, whisk together flour, baking powder, baking soda and salt. Set aside.
4. In another large bowl, cream butter and sugar on medium speed until smooth, about 3 minutes. Beat in eggs, one at a time. Add vanilla and beat until incorporated.
5. On low speed, beat in flour mixture in three additions, alternating with sour cream. Fold in chocolate.
6. Spoon half the batter into prepared pan. Sprinkle on one third of the topping and all the walnuts. Spoon remaining batter over nuts. Sprinkle remaining topping over top of cake.
7. Bake in 350 degrees F oven 55 minutes or until toothpick comes out clean. Cool on rack 20 minutes. Loosen edges, lift out cake with long spatula. Completely cool. Dust with confectioners sugar.