Heat oven to 350°F. Spray 13x9-inch pan with nonstick cooking spray. In large bowl, combine brown sugar and butter; beat until smooth. Stir in oats and cereal. Add flour, baking soda and salt; mix until well blended. Reserve 3 cups oat mixture for topping. Press remaining mixture in bottom of sprayed pan. Bake at 350°F. for 10 minutes.
Meanwhile, in small saucepan, combine caramels and milk; heat over low heat until caramels are melted and mixture is smooth, stirring constantly.
Remove partially baked base from oven. Sprinkle with 1 cup of the chocolate pieces. Drizzle with caramel mixture, being careful not to let caramel touch sides of pan. Drop reserved oat mixture by heaping teaspoonfuls over caramel mixture. Sprinkle with remaining chocolate pieces.
Return to oven; bake an additional 20 to 24 minutes or until center is set and top is golden brown. Cool 1 hour or until completely cooled. Cut into bars.