Top-ranked recipe named "Dark Chocolate Chiffon Cake"
Preheat oven to 325 degrees. Use an ungreased 10" tube pan (two piece angel food pan works great!).
Please egg whites and cream of tarter in a medium bowl. Beat with an electric mixer on high speed until stiff peaks form, 2-3 minutes. Set aside.
In large bowl mix cake mix, yolks, water, oil, cocoa, vanilla with an electric mixer on low speed for 1 minute. Scrap sides of bowl and continue mixing on medium for 2 minutes more, scraping sides as needed. Batter should be well blended.
Add egg whites to batter and fold in with rubber spatula until the mixture is light but well combined. Pour batter into tube pan. Smooth top and place in oven.
Bake for 48 to 52 minutes, cake is done when a toothpick comes out clean. Remove pan from oven and immediately turn upside down (if your pan doesn't have the extended tabs balance center on an upturned glass). Cool for 1 hour.
If using two piece pan push cake out onto rack and remove cake bottom (may need to run a knife between cake and pan). Invert again onto serving dish, so that it is right side up.
Place butter and cocoa in bowl and mix with electric mixer on low for 30 seconds.
Add confectioners sugar, milk, and vanilla. Increase mixer speed to medium and beat until light and fluffy, 1 minute or so.
Spread frosting on the top of and sides of cake with clean, smooth strokes.
Optional: Using vegetable peeler make chocolate curls with the hershey bar (along edge) and decorate on top.
Mixer should be heavy duty. Cake mix is pretty thick, a light weight hand mixer may over heat.