Position a rack in the middle of the oven and preheat the oven to 325°F Butter an 11-inch ring mold or a 12-cup (10-inch) Bundt pan, and place in a deep roasting pan.

Sift together the sugar and cocoa into a medium bowl. Shift in cinnamon, if using.

In the bowl of a stand mixer fitted with the whisk attachment, whip the whites on medium speed until soft peaks form, 5 to 7 minutes. Do not overbeat.

Meanwhile, bring a medium saucepan of water to a boil for the water bath.

Remove the bowl from the mixer, and top the whites with the cocoa mixture. This will look like a lot to incorporate, but the batter will come together. Slowly and gently fold the ingredients together, reaching to the bottom of the bowl to incorporate all the whites.

Pour the batter into the prepared pan. Spread the batter into the corners with a rubber spatula.

Pour enough boiling water into the roasting pan to come 2 to 3 inches up the sides of the mold. Bake for 45 minutes to 1 hour, or until a skewer inserted in the center of the cake comes out clean. The cake will puff up as it bakes, then shrink back as it cools.

Carefully transfer the mold to a cooling rack, and cool completely in the pan. Run a knife around the edges of the pan and unmold the cake onto a serving plate. Serve with whipped cream and, if desired, a sprinkling of cacao nibs.