Measure buttermilk, soda in liquid measuring cup, set aside. In large mixing bowl measure flours, sugar and baking powder. Blend. Add egg, oil, vanilla and buttermilk mixture. Stir to blend. TOPPING: 1/2 c lightly packed brown sugare : 1 tbsp. cinnamon : 1 tbsp. butter Spread half of batter in 9x9 greased pan and sprinkle with 1/3 of topping and cover with rest of batter. Sprinkle rest of batter on top. Bake at 350 F for 45 min. Cool on wire rack.