Double Chocolate Chiffon Cake

Directions

1. Position a rack in the lower third of the oven. Preheat the oven to 325°F (adjust for a convection oven according to the instructions with your oven). Have ready a 10-inch (10-12 cup) tube pan with a removable bottom (ungreased), and a stand mixer fitted with the whisk attachment. If the pan tube is not taller than the height of the pan sides, find a bottle with a neck thin enough to fit into the tube so that you can suspend the cake pan upside down on the bottle to cool.
2. Put the cocoa powder in a large mixing bowl (the bowl it must be large enough to mix the all of the ingredients and fold in whipped egg whites later). Whisk in the boiling water and set aside to cool for at least 10 minutes.
3. While the cocoa mixture is cooling, use a long serrated bread knife to chop the chocolate into very fine shards and pieces—the largest pieces should be not be more than 1/4 inch thick. Leave the chocolate on the cutting board until needed.
4. Set aside 1/4 cup (50 grams) of the sugar to stiffen the egg whites later.
5. Add all of the remaining sugar to the cooled cocoa mixture with the baking powder, baking soda, salt, oil, egg yolks, and vanilla. Whisk until the ingredients are well blended. Add the flour and whisk until blended and smooth. Set aside.
6. In a stand mixer fitted with a whisk, beat the egg whites and cream of tartar on medium-high speed until the mixture is creamy white—no longer yellow or translucent—and holds soft peaks when the beaters are lifted. Gradually beat in the reserved 1/4 cup of sugar, increase the speed to high and beat briefly until the egg whites are stiff but not dry.
7. Use a large rubber spatula to fold one quarter of the egg whites in to the cocoa batter. Scrape the remaining egg whites into the bowl and scrape the chocolate from the cutting board over the top. Fold the egg whites and chocolate into the batter.
8. Scrape the batter into the pan. Bake 55-60 minutes, or until a tooth pick inserted in a couple of places comes out clean and dry.
9. If the tube is taller than the sides of the pan, simply turn the pan upside down (supporting the tube as you do so) and let it rest on the tube (the cake will be suspended an inch or two above the counter). If the tube is not tall enough to suspend the cake, insert the neck of a bottle into it and carefully turn the pan upside down, holding on to the bottle, and let it rest and balance on the bottle (which is now right side up). Leave the cake suspended upside down to cool.
10. To remove the cake from the pan, turn it right side up and slide a thin metal spatula around the sides, pressing against the pan to avoid tearing the cake. Detach the cake from the tube with the spatula or with a skewer. Lift the tube to remove the cake from the pan. Detach the cake from the bottom of the pan by sliding the spatula between the cake and the pan, around the tube, pressing against the pan bottom as you go. Lift the cake off of the tube with two spatulas (one on either side of the tube) and transfer it to a platter. I like to keep it right side up, but you can turn it upside down if you prefer.
11. You can sprinkle the cake with a little powdered sugar (or not). Slice with a sharp serrated knife and serve it plain, or with dollops of whipped cream and berries, or with ice cream and chocolate sauce.