Preheat oven to 350 degrees. Line 24 standard muffin cups with paper baking cups.
Prepare cake mix according to package directions. Stir in 3/4 cup chocolate chips and 3/4 cup walnuts. Spoon batter into prepared muffin cups, filling 2/3 full.
Bake 20 minutes or until toothpick inserted into center comes out clean. Cool in pans for 10 minutes, then remove to wire racks to cool completely.
Combine frosting and peanut butter in medium bowl until well blended. Frost cupcakes, sprinkle with remaining 3/4 cup chocolate chips and 3/4 cup walnuts.