Preheat oven to 300°F. Spray a 9-inch round springform pan with non-stick cooking spray and line with parchment paper.
In a small saucepan over low heat, melt the butter and chopped chocolate, whisking until completely melted and smooth. Remove from heat and transfer to a large bowl. Set aside to cool.
In the bowl of a stand mixer fitted with the whisk attachment, or in a large bowl with a hand mixer, beat egg whites on medium speed just until foamy, about 30 seconds.
While continuing to beat on medium-high speed, slowly add in the sugar. Eggs will begin to become glossy and bright white.
Continue to beat on medium-high until stiff peaks form on the end of the whisk. The meringue should be stiff and not drip off of the whisk attachment. This took me about 5-7 minutes of mixing.
Add the egg yolks and vanilla to the slightly cooled chocolate mixture and whisk together until smooth.
Sift the dutch processed cocoa powder and espresso powder into the chocolate and egg mixture. Add the salt and whisk until combined.
Add ⅓ of the egg whites to the chocolate mixture and gently fold using a large spatula. Continue to add the egg white meringue to the chocolate mixture and gently fold until no streaks remain. Be sure not to over mix because you will lose the airy texture of the egg whites and will have a flat cake!
Pour mixture into the prepared pan, smoothing top with an offset spatula.
Bake in on the center rack for 30-40 minutes, depending on how powerful your oven is. Watch closely and remove the cake when it is set on the edges and in the middle. You will not want the cake to be jiggly in the middle. Do not overbake or the cake will be dry!
Remove from oven and cool on a wire rack for 30 minutes to 1 hour.
Remove the springform pan and continue to cool on a wire rack until room temperature. Sometimes I like to stick the cake in the fridge before topping with whipped cream and slicing because it helps the cake stay together better when cutting it!
Top with Espresso Whipped Cream (instructions below).