1Heat oven to 350°F. In large bowl, stir together cookie mix, cocoa and flour. Add sour cream, butter, vanilla and egg; stir until stiff dough forms.
2Shape dough into 28 (1 1/4-inch) balls. Roll in sugar. On ungreased cookie sheets, place balls 2 inches apart. Press each ball to flatten slightly.
3Bake 8 to 9 minutes or until set (do not overbake). Cool 2 minutes; remove from cookie sheets to cooling racks. Cool completely, about 15 minutes.
4In small bowl, stir filling ingredients until well mixed. To make each whoopie pie, spread about 2 teaspoons filling on bottom of 1 cookie. Top with second cookie, bottom side down; gently press cookies together. Store between sheets of waxed paper in tightly covered container.