PREHEAT oven to 350°F. Mix 2 pkt. of the cereal, the flour, baking powder, baking soda and salt; set aside. Beat 1/2 cup of the margarine and the brown sugar in large bowl with electric mixer on medium speed until well blended. Add egg; mix well. Add flour mixture alternately with the buttermilk, beating well after each addition. Spread into greased 13x9-inch baking pan.
MIX remaining 3 pkt. of cereal and remaining 1/4 cup margarine until well blended; sprinkle over batter.
BAKE 40 to 45 minutes or until toothpick inserted in center comes out clean. Cool slightly. Drizzle with Powdered Sugar Glaze. Cut into 24 squares. Serve warm.