1Heat oven to 350°F. Line cookie sheets with cooking parchment paper, or lightly spray with cooking spray.
2In large bowl, beat cookie ingredients with electric mixer on low speed until moistened; beat 1 minute longer on high speed. Drop batter by 2 tablespoonfuls 2 inches apart onto cookie sheets.
3Bake 8 to 11 minutes or until set (do not overbake). Cool 2 minutes; remove from cookie sheets to cooling racks. Cool completely, about 15 minutes.
4In large bowl, beat filling ingredients with electric mixer on high speed until light and fluffy. For each whoopie pie, spread about 2 tablespoons filling on bottom of 1 cooled cookie. Top with second cookie, bottom side down. Store covered in refrigerator.