If you don't have 2 jelly-roll pans, use 2 (15- x 10 1/4-inch) disposable foil pans.
Line 2 (15- x 10-inch) jelly-roll pans with parchment or wax paper. Prepare angel food cake mix batter according to package directions, adding unsweetened cocoa. Pour evenly into prepared pans.
Bake at 325° for 15 to 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pans on wire racks 10 minutes. (If baking cakes in 1 oven, bake on middle 2 racks for 10 minutes; then switch places, and continue baking for 5 to 10 minutes.)
Sift 2/3 cup powdered sugar evenly over 2 (24- x 18-inch) pieces heavy-duty aluminum foil.
Loosen edges of cakes from pans. Invert each slightly warm cake onto a prepared foil piece. Carefully remove parchment paper, and discard. Place a cloth towel on top of each cake. Starting at 1 long side, roll up foil, cake, and towel together.
Chill rolled cakes 30 minutes or until completely cool. Unroll cakes, and remove towels. (Keep each cake on foil piece.)