Directions

1. Preheat the oven to 350° F. and position a rack in the center of the oven.

2. Prepare the pan. Using a pastry brush, spread 1 Tbsp. of the softened butter over the bottom and side of a 9-inch springform pan and sprinkle with 1 Tbsp. cocoa powder to lightly coat it. Holding the pan over the sink, move it in a circular pattern to distribute the cocoa evenly over the buttered surface, discarding any excess into the sink. Set aside.

3. 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 45-60 seconds, until the butter is completely melted. Let cool for 2 to 3 minutes until the chocolate is warm, but not hot.

4. 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.

5. In the perfectly clean bowl of a stand mixer fitted with the whisk attachment (or use a mixing bowl and an electric mixer or whisk), beat the egg whites at low to medium speed for about 15-20 seconds (or a little longer with whisk or electric mixer), or until they are broken up and foamy.

6. Add the cream of tartar and beat at medium speed, for 45 to 90 seconds (or a little longer if using a handheld mixer, at medium-high speed for a little longer) until the whites form fluffy, droopy peaks when the beater is lifted.

7. With the mixer still at medium speed, gradually add the sugar and beat until you create 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 the mixer and other factors, so you will need to rely on your eyes to be sure the mixture has reached the right texture. This is not the time to walk away from the mixer.

8. 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.

9. 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 ½ hours. Remove the side of the pan and serve.