Preheat the oven to 350 F and place a rack in the center of the oven.
Add 2 inches of water into a small pot or the base of a double boiler. Place the pot on the stove and bring the water to a simmer.
Put the chocolate and butter into a metal mixing bowl that will fit over top of a sauce pot or the bowl of a double boiler.
Place the bowl over the pot of simmering water, and stir the chocolate and butter until melted.
Alternatively you could melt the chocolate and butter in a microwave safe bowl in the microwave in 15-20 second bursts, stirring in between.
Set the bowl with the melted chocolate and butter aside to cool slightly.
Separate 6 eggs adding the whites into the bowl of a stand mixer or a large mixing bowl and the yolks into a small bowl. Add the sugar and salt into the bowl with the egg whites - whip until stiff peaks form.
Add the egg yolks and vanilla into the bowl with the melted chocolate and butter and whisk to combine.
In three additions, fold the whipped egg whites into the chocolate mixture. Careful not to stir and knock the air out of the egg whites.
I like to fold in the first addition with the whisk attachment from my stand mixer but by hand and then swap to a flat rubber spatula for the next two additions.
Line a 9" springform pan with a piece of parchment paper, it's easiest if you crumple and then smooth out the parchment.
Pour the fluffy chocolate batter into the pan. Bake it in the hot oven for 45 minutes until the entire surface has puffed and cracked. It will still jiggle slightly- that's ok.
Take the cake out of the oven and allow it to cool for an hour before removing it from the pan. The cake will sink as it cools, that's what is supposed to happen so don't let it stress you out.