10 tablespoons plus 1 tablespoon unsalted butter, room temperature, divided
2/3 cup unsweetened cocoa powder, plus 1 tablespoon for dusting
10 1/2 ounces bittersweet or semisweet chocolate (50 to 64 percent cacao), finely chopped
5 large eggs, separated (see Recipe Notes)
1 egg yolk
1/2 teaspoon espresso powder
1/4 teaspoon salt
1 teaspoon pure vanilla extract
1/2 teaspoon cream of tartar
1 cup granulated sugar
9-inch springform pan
Microwave-safe bowl and mixing bowl
Stand mixer with whisk attachment or handheld electric mixer
- Preheat the oven: Preheat the oven to 350°F and position a rack in the center of the oven.
- Prepare the pan: Using a pastry brush, spread 1 tablespoon of the softened butter over the bottom and side of a 9-inch springform pan and sprinkle with 1 tablespoon cocoa powder to lightly coat it. Holding the pan over the sink, move it in a circular motion to distribute the cocoa evenly over the buttered surfaces, discarding any excess into the sink. Set aside.
- Melt the chocolate: Place the remaining butter and the chocolate in a large microwave-safe bowl and microwave for 15 seconds. Stir the mixture. Repeat the process, heating the mixture 2 or 3 more times for 15 seconds at a time, stopping to stir each time, for a total of 45 to 60 seconds, until the butter is completely melted. Let cool for 2 to 3 minutes until the chocolate is warm, but not hot.
- Combine the chocolate and other ingredients: Whisk the egg yolks into the cooled chocolate mixture. Sift in the remaining cocoa powder, the espresso powder, and salt. Add the vanilla extract and whisk until completely combined.
- Begin to beat the egg whites: In the perfectly clean bowl of a stand mixer fitted with a perfectly clean whisk attachment (or if you are using a handheld electric mixer or a wire whisk — use a large, perfectly clean mixing bowl), beat the egg whites at low to medium speed for about 15 to 20 seconds (or a little more for a handheld mixer or whisk), until they are broken up and foamy.
- Add the cream of tartar: Add the cream of tartar (see Recipe Notes) and beat at medium speed, for 45 to 90 seconds (or if you are using a handheld mixer, at medium-high for a little longer) until the whites form fluffy, droopy peaks when the beater is lifted.
- Beat to form stiff peaks: With the mixer still at medium speed, gradually add the sugar and beat until you’ve created glossy, stiff peaks that stand by themselves when you lift the beaters slightly, 1 to 2 minutes. The exact timing will depend on the size and shape of the bowl, the power of your mixer, and other factors, so you will need to rely on your eye to be sure they have reached the right texture. This is not a time to walk away from the mixer.
- Fold the whites into the batter: Whisk about a quarter of the whipped egg whites into the chocolate mixture. Then, with a silicone spatula, gently scoop the remaining egg whites into the batter. With the same spatula, scoop them from underneath, along with some of the batter, and very gently turn the whole thing over; repeat until just incorporated.
- Bake the cake: Pour the batter into the prepared pan and smooth the top with a silicone spatula. Bake for 34 to 37 minutes, until the cake has risen ever-so-slightly in the center and is set enough in the center that it will not jiggle when the pan moves; a cake tester will not come out clean. The top will have formed an extremely thin, but discernible, crust. The cake will pull away from the side of the pan while it cools; don’t let it bake until it pulls away from the side of the pan while in the oven or it will be overdone. Cool completely on a wire rack for at least 1 1/2 hours. Remove the side of the pan and serve.
- Eggs separate better when they are cold, rather than at room temperature, so separate them right after removing them from the fridge. This is key in any recipe that involves whipping egg whites (like this one) because if you get any yolk mixed in with your whites, they won’t whip as well as they might.
- In case you are wondering, the cream of tartar stabilizes the egg whites.
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