Heat oven to 350. Spray 24 muffin tins w/ cooking spray, or use liners to line muffin cups. Make cake as directed, stirring in peanut butter chips. Spoon batter into muffin cups, 3/4-full. Bake as directed, cool.
In lg bowl, beat cream cheese, butter and peanut butter until smooth. Beat in powdered sugar and vanilla until smooth. Frost cooled cupcakes.
Drizzle cupcakes w/ hot fudge sauce. Refrigerate until chocolate is set.