For Cupcakes: HEAT oven to 350°F. Line 24 muffin cups with paper liners.
BEAT cake mix, water, oil and eggs in large bowl with electric mixer set on medium speed until moistened. Beat 2 minutes. Divide evenly into muffin cups.
BAKE 18 to 22 minutes or until toothpick inserted in center comes out clean. Cool 5 minutes. Remove from pan. Cool completely on wire rack.
For Filling: MIX peanut butter and butter in medium bowl until smooth. Add powdered sugar and milk and beat until smooth and creamy.
REMOVE cupcakes from papers. Cut in half crosswise using a serrated knife. Spread about 1 tablespooon filling on bottom halves. Frost top halves as desired with chocolate frosting. Place frosted tops over filling and press gently. Sprinkle with peanuts.