1 1/2stickunsalted butter12 tablespoons, plus more for the pan
Preheat the oven to 350°F. Cut a circle of parchment paper to fit in the bottom of a 9 inch springform pan. Lightly butter the pan and the parchment paper.
Combine the butter and chocolate in a heat proof bowl over simmering water *don't let the water touch the bottom of the bowl) whisking until smooth.Alternately, you could melt the chocolate and butter in the microwave in 30 second intervals, stirring in between each time. Set aside to cool slightly.
Add the eggs, sugar, salt and vanilla to the bowl a stand mixer fitted with the whisk. Whisk for 5-8 minutes, until pale, thick and doubled in size.
Add 1/3 of the egg mixture to the chocolate, carefully folding it in. Once mixed, another 1/3, folding it in. Add the chocolate mixture to the remaining egg mixture, folding until mixed. Pour the batter into the prepared pan. Bake in the oven for 35-45 minutes, or until top is not shiny and center doesn't jiggle. Allow to cool completely before cutting and serving.
Because this cake gets it's structure from eggs, it will deflate as it cools and the top will be crackly - that is normal!If you don't have a stand mixer, you can use a hand mixer. You could even do this by hand with a whisk, although it will take some arm muscles and time!Cake can be made one day ahead of time and stored in an airtight container in the fridge. Allow to come to room temperature before serving.Store leftovers in an airtight container in the fridge for 2-3 days.