Preheat oven to 350°.
Combine first 3 ingredients in a large bowl; beat with a mixer at medium speed for 2 minutes or until mixture resembles damp sand. Add eggs, 1 at a time, beating well after each addition. Stir in vanilla.
Stir together flour, baking powder, and salt in a bowl. Gradually add flour mixture, beating until blended. Stir in 3 tablespoons chocolate chips and butterscotch chips. Spread batter evenly into an 8-inch square baking pan coated with cooking spray. Sprinkle batter with nuts; lightly press into batter with fingers. Bake at 350° for 35 minutes or until a wooden pick inserted into center comes out clean. Cool in pan on a rack. Cut into 16 2- to 3-inch squares.
Place remaining 1 tablespoon chips in a small heavy-duty zip-top plastic bag; seal. Microwave on high 1 minute or until melted. Snip a hole in 1 corner of bag; drizzle chocolate over bars.