Line 2 lightly greased 15- x 10-inch jellyroll pans with wax paper; lightly grease paper.
Beat eggs in a large bowl at medium-high speed with an electric mixer 5 minutes. Add 1/2 cup water, beating at low speed just until blended. Gradually add cake mix, beating at low speed just until moistened. Beat at medium speed 2 minutes. Divide batter in half; spread evenly into prepared pan. (Layers will be thin.)
Bake each cake at 350° on the middle rack in separate ovens for 13 minutes or until cake springs back when lightly touched in the center. (If you don't have a double oven, set 1 pan aside.)
Sift 1 to 2 tablespoons cocoa in a 15- x 10-inch rectangle on a cloth towel; repeat with second towel. When cakes are done, immediately loosen from sides of pans, and turn each out onto a prepared towel. Peel off wax paper. Starting at narrow end, roll up each cake and towel together; place cakes, seam sides down, on wire racks. Cool cakes completely.