Combine quarter cup brown sugar and next 4 ingredients in a small bowl; stir well, and set aside. Combine remaining cup brown sugar and oil in a large bowl, beating well at medium speed of an electric mixer. Add egg whites; beat well. Combine flour and next 3 ingredients; add to brown sugar mixture alternately with buttermilk, beginning and ending with flour mixture. Mix after each addition. Stir in vanilla. Reserve 2 cups of batter, and set aside. Pour remaining batter into a 12-cup Bundt pan coated with cooking spray. Spoon pumpkin mixture evenly over center of batter to form a ring. Pour reserved batter over pumpkin mixture. Bake at 180ºC for 50 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes; remove from pan. Cool on a wire rack. Combine powdered sugar and ----