Preheat the oven to 350°F (or 325°F if using dark or nonstick cookie sheets). Prepare the cookie sheets by lightly coating them with cooking spray or lining them with parchment paper.
In a large mixing bowl, combine all the cookie ingredients. Begin mixing on low speed with an electric mixer until everything is moistened. Increase the speed to high and continue mixing for about 1 minute, scraping the sides of the bowl occasionally. Drop the batter onto the prepared cookie sheets using 2-tablespoon portions, spacing them about 2 1/2 inches apart.
Bake the cookies for 10 to 12 minutes, or until they are set but not overbaked. Allow the cookies to cool on the sheets for 2 minutes before transferring them to a cooling rack to cool completely, approximately 30 minutes.
To assemble each whoopie pie, spread roughly 3 tablespoons of the frosting onto the flat side of one cooled cookie. Place a second cookie on top, with its flat side down, creating a sandwich. Store the whoopie pies in a single layer in a covered container.