Preheat oven to 350 degrees.
Coat 2 15x10x1-inch jelly roll pans with cooking spray;line with wax paper, and coat with spray. Set aside
Beat eggs in a large mixing bowl at medium speed 5 minutes. Add water,beating at low speed just until blended. Gradually add package of cake mix,beating at low speed just until moistened. Beat mixture at medium speed 2 minutes. Divide batter in half, and spread batter evenly into prepared pans.
Bake each cake on the middle rack for 13 minutes or until cake springs back when lightly touched in the center. May need to bake cakes separately.
Sift 2 tablespoons cocoa in a 15-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,seam side down, on wire racks,to cool completely.
In a heatproof bowl whisk together yolks, sugar, cornstarch, cocoa powder, and salt (mixture will be very stiff). In a heavy saucepan bring milk just to a boil over moderate heat and in a stream add 1/4 cup to egg mixture, whisking until smooth, and whisk in remaining 3/4 cup milk. Transfer egg mixture to pan and bring to a boil, whisking (mixture will look curdled but will become smooth as whisked). Boil pastry cream, whisking vigorously, 1 minute and remove from heat. Stir in chocolate and butter, stirring until melted and combined well. Transfer hot pastry cream to a heatproof bowl and chill, surface covered with plastic wrap, until cold, about 2 hours.
Unroll each cake roll,fill with pastry cream,re-roll. Sprinkle one with powdered sugar,the other with cocoa