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" round cake pan coated with cooking spray.
Sprinkle reserved 1/2 cup flour mixture over batter.
Bake at 350º for 30 minutes or until cake springs back
when touched lightly in center. Cool on a wire rack.