Preheat oven to 350 F. Grease an 9" springform pan with butter. Dust the sides of the pan with cocoa powder. Cut a piece of parchment to fit the bottom of the pan. Cut an additional piece of parchment to collar, or go around the inside of the pan, an inch higher than the height of the pan.
In the bowl of a food processor, combine the cocoa powder, sugar and chopped chocolate. Process the mixture to pulverize the chocolate.
¼ cup cocoa powder, 1 ¾ cups bittersweet chocolate, chopped or chips, 1 cup granulated sugar
Pour the hot coffee slowly into the feed tube while running the processor on low speed. Process until the chocolate is completely melted and the mixture is smooth.
¾ cup coffee, hot, freshly brewed
Separate the eggs, placing the egg yolks into a small bowl, and placing the whites into the bowl of a stand mixer.
6 large eggs
Process the chocolate mixture again, on low, streaming in the melted butter, egg yolks, salt and vanilla until smooth.
1 cup unsalted butter, melted and cooled, ¾ teaspoon fine sea salt, 2 teaspoons pure vanilla extract
Transfer the chocolate mixture to a large bowl, and set aside.
Meanwhile, place the bowl with the egg whites onto the stand mixer, fitted with a whisk attachment.
Beat the whites on medium high speed until stiff peaks form.
Carefully fold the egg whites into the chocolate mixture with a spatula. This can take a few minutes to ensure everything is well incorporated.
Pour the cake batter into the prepared pan. Bake the cake for 30 minutes, before adding the meringue.*If you are not making this cake with meringue, bake the cake for 50-60 minutes, until the top is puffed and dry. The internal temperature should read over 200F when done.* In a medium bowl, whisk together the brown sugar and espresso powder (if using).
⅔ cup light brown sugar, packed, 1 teaspoon espresso powder
In the bowl of a stand mixer fitted with the whisk attachment, place the egg whites and the salt.
3 large egg whites, ⅛ teaspoon fine sea salt
Beat the whites on medium speed until soft peaks form.
Beat in the sugar/espresso powder a tablespoon at a time. When all the sugar has been added, beat until stiff peaks form.
Remove the bowl from the mixer. Fold in the vanilla, vinegar and cornstarch.
1 teaspoon pure vanilla extract, 1 teaspoon apple cider vinegar, 2 teaspoons cornstarch
After the cake has baked for 30 minutes, remove it from the oven. Top the cake with the meringue, carefully spreading it evenly over the surface of the cake.
Return the cake to the oven and bake for an additional 25-30 minutes.
Push the meringue back into the pan with an offset spatula as the meringue will deflate and crack as it cools. Let the cake cool for 15 minutes in the pan.
Remove the cake from the sides of the springform pan. Let the cake cool to room temperature before serving.