Place egg whites in a large bowl; set aside for 30 minutes. Meanwhile, stir cocoa and water in a small bowl until smooth. Cool.
Place oven rack in lowest position. Preheat oven to 325°. In a large bowl, combine flour, sugar, baking soda and salt. In another bowl, whisk oil, egg yolks, vanilla and cocoa mixture. Add to dry ingredients; beat until well blended. Add cream of tartar to egg whites; with clean beaters, beat on high speed just until stiff but not dry. Fold a fourth of the whites into batter, then fold in remaining whites.
Gently spoon into an ungreased 10-in. tube pan. Cut through batter with a knife to remove air pockets. Bake 60-65 minutes or until cake springs back when lightly touched. Immediately invert pan; cool completely, about 1 hour.
Run a knife around side and center tube of pan. Remove cake to a serving plate. For filling, in a large bowl, combine all ingredients until smooth; refrigerate for 1 hour.
Beat until stiff peaks form. Cut a 1-in. slice off the top of the cake; set aside. To make a tunnel, carefully hollow out bottom, leaving a 1-in. shell. Fill tunnel with some of the filling; replace cake top. Spread remaining filling over top and sides of cake. Store in the refrigerator.