PREHEAT oven to 350° F. Grease 13 x 9-inch baking pan.
COMBINE cake mix and peanuts in large bowl; stir in butter. Stir in evaporated milk (batter will be thick). Spread half of batter into prepared 13 x 9-inch baking pan.
BAKE for 15 minutes.
MICROWAVE 1 1/3 cups morsels and peanut butter in medium, uncovered, microwave-safe bowl on MEDIUM-HIGH (70%) power for 1 minute; STIR. The morsels may retain some of their original shape. If necessary, microwave at additional 10- to 15-second intervals, stirring just until morsels are melted. Spread mixture over baked layer. Drop remaining batter by heaping teaspoon over butterscotch mixture.
BAKE for 20 to 25 minutes (top layer will be soft). Cool completely in pan on wire rack.
PLACE remaining morsels in small, heavy-duty plastic bag. Microwave on MEDIUM-HIGH (70%) power for 30 seconds; knead. Microwave at additional 10- to 15-second intervals, kneading until smooth. Cut tiny corner from bag; squeeze to drizzle over brownies.