Chiffon cake has a texture similar to that of sponge cake. Although baking powder aids the leavening, for the tallest cake, gently fold the grated chocolate into the cake batter and distribute it evenly.
Heat oven to 325°F. Sift together cake flour, sugar, baking powder and salt into large bowl. Whisk oil, egg yolks, water, vanilla and lemon juice in second bowl. Stir oil mixture into flour mixture until smooth.
Beat whites and cream of tartar in clean bowl on medium speed until foamy. Increase speed to high; beat until stiff peaks, about 3 minutes. Fold one-third of whites into flour mixture. Fold in remaining whites and grated chocolate. Gently scrape into ungreased 10-inch tube pan.
Bake in 325°F oven 55 to 60 minutes or until top springs back when lightly pressed with fingertip. Invert pan; let cake cool completely.
To serve, run thin spatula around edge of pan and center tube. Remove cake from pan; place on cake pedestal. Drizzle melted chocolate over top; let stand to harden.