I know there are already some awesome recipes on here for these, but these are a N.E. tradition and are always a big hit with anyone that loves whoopee pies. I use coconut oil because it's healthier and it imparts a nice flavor. But you can use butter instead.
Heat oven to 350. In medium bowl, beat cake mix, pumpkin puree and melted coconut oil. Beat in water. Lay parchment paper on lg cookie sheets.
Using a scant 1/4 cup, place 24 equal amounts of batter onto parchment paper, about 1' apart. Bake about 12 mins, or until tested with a toothpick.
Let cool. In another bowl, Beat together cream cheese and melted white chocolate. Pipe or spoon onto flat side of half of the cakes. Top w/ remaining cakes. Sprinkle w/ a little powdered sugar. Wrap individually in plastic and refrigerate until serving.