Preheat oven to 160° C (325° F). Dissolve coffee powder or granules in hot water; cool to room temperature. In a large bowl, combine flour, sugar, baking powder, and salt; stir until well mixed. Make a well in center; add oil, egg yolks, vanilla and cooled coffee. Beat with a wooden spoon until smooth. In another large bowl, use electric mixer to beat egg whites and cream of tartar until stiff but not dry. Pour batter over egg whites; fold in carefully, adding grated chocolate towards the end. Spoon batter into an ungreased 10 inch (25 cm) tube pan. Bake 60 to 70 minutes, or until cake springs back when lightly touched. Turn pan upside down and let cake hang until cool. Remove from pan.
Cocoa Whipped Cream
In a medium bowl, combine whipping cream, sugar, cocoa powder and vanilla. Stir to blend ingredients, but do not beat. Cover and refrigerate 1 to 2 hours. Whip until soft peaks form. Frost cake with cocoa whipped cream. Decorate with chocolate eggs or semi sweet chocolate curls. Refrigerate until ready to serve.