Preheat oven to 350°.
Lightly spoon the flour into dry measuring cups, and level with a knife. Combine the flour, brown sugar, cinnamon, and salt in a bowl, and cut in margarine with a pastry blender or 2 knives until the mixture resembles coarse meal. Reserve 1/2 cup flour mixture for topping, and set aside.
Combine remaining flour mixture, baking powder, and baking soda, and add the buttermilk, vanilla, and egg. Beat at medium speed of a mixer until blended. Spoon batter into an 8-inch round cake pan coated with cooking spray. Sprinkle reserved 1/2 cup flour mixture over batter. Bake at 350° for 30 minutes or until cake springs back when touched lightly in center. Cool on a wire rack.