Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper or a silpat liner.

In a large bowl, mix together the cake mix, apple butter, oil and eggs until smooth. Let stand for 3-4 minutes.

Spoon the batter onto the prepared baking sheet, a heaping rounded tablespoon at a time. Space them about 2 inches apart. (Using a small scoop will keep the size consistent.)

Bake for about 14 minutes and let them cool completely.

Frost the flat side of each "pie" half with either the marshmallow fluff (see Note below) or the cream cheese frosting, and place halves together.

To make the cream cheese frosting, beat the cream cheese, butter and powdered sugar until fluffy. Add the milk and vanilla, and beat for another 1-2 minutes.

Store whoopie pies in sealed container in refrigerator.