Whip the egg whites with cream of tartar until stiff peaks form.
Whisk together cake flour, cocoa powder, baking powder, salt, and sugar in a large bowl.
Blend egg yolks, oil, vanilla, and water in a separate bowl until smooth. Pour into the dry mixture and whisk until combined.
Gently fold the whipped egg whites into the batter in three additions.
Pour batter into an ungreased Bundt or tube pan and bake at 325°F (165°C) for 50–55 minutes. Cool upside down before removing from the pan. Decorate with powdered sugar, chocolate candies, and festive toppings.
Notes
Do not grease the pan to help the cake rise properly.
Cooling upside down prevents the cake from collapsing.