1Heat oven to 375°F. In large bowl, stir together cookie mix and flour. Add remaining cookie ingredients; stir until stiff dough forms.
2Onto ungreased cookie sheets, drop dough by 36 rounded teaspoonfuls 2 inches apart. Lightly press tops with floured fingertips to flatten slightly.
3Bake 8 to 10 minutes or until set. Cool 2 minutes; remove from cookie sheets to cooling racks. Cool completely, about 15 minutes.
4In medium bowl, beat filling ingredients with electric mixer until light and fluffy. For each whoopie pie, spread about 2 teaspoons of the filling on bottom of 1 cooled cookie. Top with second cookie, bottom side down; gently press together. Store tightly covered in refrigerator. Sprinkle with additional powdered sugar just before serving.