How to Make It
Preheat oven to 350°F. Grease a 9-inch springform pan with butter and line the bottom with a 9-inch circle of parchment paper. Set aside.
Combine chocolate, butter, and espresso powder over a double boiler. Stir together until fully melted and incorporated. Remove from heat and set aside.
With the whip attachment on a stand mixer, whip together eggs, sugar, and salt on medium-high until quadrupled in volume, about 6 minutes.
Transfer egg mixture to a large mixing bowl. Add half of melted chocolate mixture and gently stir to combine. Add remaining chocolate and fold to fully combine.
In stand mixer, beat cream to medium peaks (Note: There’s no need to clean the bowl and whisk attachment from the egg mixture). Fold whipped cream into chocolate mixture. Carefully transfer batter to prepared springform pan. Bake until the surface doesn’t move when shaken and a toothpick inserted comes out clean, about 45 minutes. Allow to cool completely before removing from pan. Dust with cocoa powder and serve.
If you don’t want to bother with a double boiler, simply melt the chopped chocolate in the microwave by heating it on 30% power for 30 second intervals, stirring between each interval, until fully melted. Melt the butter separately and then stir into melted chocolate. Proceed with recipe as written.