Heat oven to 350 degrees. Line a 9-inch square pan with foil; coat foil with nonstick cooking spray. Prepare Topping: In a bowl, blend brown sugar, flour, cinnamon and salt. Work butter into flour mixture with your fingertips until crumbly. Refrigerate while preparing cake batter.
Cake: Combine flour, baking powder, baking soda and salt. Set aside. Beat butter and sugar until smooth, 2 to 3 minutes. Beat in eggs, one at a time. Mix in vanilla.
On low speed, beat in half the flour mixture, the buttermilk and then remaining flour mixture. Spread half the batter into prepared pan. Top with 1 cup of the topping, finely crumbling to cover batter. Top with remaining batter, spreading to cover topping. Crumble remaining 2 cups topping over batter.
Bake at 350 degrees for 55 to 65 minutes or until toothpick inserted in center comes out clean. Cool in pan on a wire rack.