1In a medium mixing bowl with mixer on low speed, cream peanut butter until soft. Gradually add milk while continuing to mix. (It is important to add milk slowly; if added too fast, peanut butter will harden.)
2When all of the milk is incorporated with the peanut butter, add pudding mix and continue to blend until all ingredients are smooth and thoroughly combined, about 1 1/2 minutes.
3Pour mixture into baked pastry shell. Chill at least 4 hours before serving.