Heat oven to 350. Grease and flour 13 x 9 inch pan. In large bowl, mix cake mix, brown sugar, and peanut butter at low speed til crumbly. Reserve 1/2 cu peanut butter mixture; set aside. To remaining peanut butter mixture, add water, oil, and eggs. Mix at low speed til moistened; beat 2 minutes at highest speed. Fold 1/4 cup of the chocolate chips and 1/4 cup of the peanut butter chips into batter. Pour batter into prepared pan. In small bowl, mix reserved 1/2 cup peanut butter mixture, remaining chocolate chips and peanuts
Bake at 350 for 40 to 50 minutes or til toothpick inserted in center comes out clean. Immediately sprinkle with remaining peanut butter chips; gently press into top of cake. Cool completely.
Tip: semi sweet chocolate chips can be substituted for the peanut butter chips if desired.