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.
