Preheat the oven to 350°F or 180°C. Line the cake pan with parchment paper and set aside.
Separate the egg whites from the yolks by cracking an egg on the rim of a large bowl. Let the whites drip into the bowl and then place the yolk in a separate small bowl. Repeat with the two remaining eggs and set the bowls aside.
3 large eggs
Add the chocolate and butter to a large, heat-proof bowl. Microwave at 30 second increments until the chocolate is mostly melted. Whisk together until smooth.
¾ cup bittersweet chocolate, 6 tablespoon unsalted butter
Whisk the cocoa powder, espresso powder, vanilla, and sea salt into the chocolate mixture. Set aside to cool.
⅓ cup dutch-processed cocoa powder, 1 teaspoon espresso powder, 1 teaspoon vanilla extract, ½ teaspoon sea salt
With an electric hand mixer on medium speed, beat the egg whites in the large bowl until foamy. Gradually add the granulated sugar and beat on high speed until a meringue with stiff peaks forms. Set aside.
½ cup granulated sugar
Once the chocolate mixture has cooled, whisk in the egg yolks until fully combined.
Gently fold in the meringue in 3 additions. Try not to overmix, but mix until there are no white streaks of meringue remaining.
Gently pour the batter into the prepared cake pan and smooth out the top.
Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean with a few moist crumbs.
Let the cake to cool for about 20 minutes in the pan on a wire cooling rack.
Remove the cake from the pan, sprinkle with powdered sugar, slice, and enjoy.