COMBINE cereal, 1 cup of the brown sugar and 1 tsp. of the cinnamon; set aside. Mix together baking mix, remaining 1/2 cup brown sugar, 1-1/4 cups of the milk, eggs and oil just until blended. Pour half of the batter into a 13x9-inch baking pan sprayed with cooking spray; sprinkle with half of the cereal mixture. Repeat layers.
BAKE at 350ºF for 35 min. or until toothpick inserted in center comes out clean. Cool 10 min. Cut into 24 squares.
WHISK together powdered sugar, remaining 1/2 tsp. cinnamon and remaining 2 Tbsp. milk until well blended. Drizzle over coffee cake. Let stand until set.