Preheat oven to 350 Prepare your springform pan (I use a 9") by lightly greasing then lining with parchment paper. I baked in a water bath (wrapped the springform pan with tin foil to seal it in, then bake in about 1" of water), but you could skip the water bath also.
Melt 10 oz of semisweet chocolate that has been chopped up in a bowl. You'll want to use a bowl big enough because you'll be adding the whipped up eggs into the chocolate -- I use a big bowl. You can do this on the stove or more simply in the microwave.
Put the egg whites and about a 1/3 of the sugar into a large bowl. Start beating the egg whites with an electric mixer, and get the eggs to the point they are starting to get fluffy. Add the remaining sugar, salt and vanilla. Keep beating until the meringue is nearly at the soft-peak stage.
Whisk the softened butter into the melted chocolate and combine well. Whisk in egg yolks until combined. Add half of the beaten egg whites into the chocolate until combined. Once combined, fold in the remaining meringue.
Once combined, pour the batter into the springform pan. You'll bake for 26-28 minutes, if you use a convection oven, or quite a bit more if you aren't using convection (50 minutes or so without convection). Bake until a toothpick inserted comes out clean.
Remove the cake and allow to cool to room temperature. Refrigerate for at least an hour once cooled, and the cake will settle down significantly. Once cooled, remove the cake from the fridge, run a knife around the edges, and remove the springform ring. Flip the cake over onto your serving dish and remove the bottom and parchment paper. Place the springform ring back over the cake, you'll need it to hold the mousse in place later.