HEAT oven to 350°F.
PREPARE cake batter and bake in 24 paper-lined muffin cups as directed on package. Cool 30 min. (Cupcakes still need to be warm to fill.)
BEAT pudding mix and milk with whisk 2 min. Add peanut butter; mix well. Spoon into small freezer-weight resealable plastic bag; seal bag. Snip off one corner from bottom of bag. Insert tip of bag into center of each cupcake; pipe in about 1 Tbsp. filling.
FROST each cupcake with Frosting; sprinkle with nuts.