Melt the chocolate over a double boiler, melt the butter in a small pot, thrn combine both.
Let the mixture cool down for 5 minutes at room temperature. Beat the eggs and powdered sweetener for 2 minutes with a mixer.
Add salt and vanilla. Beating on slow speed, add in the cooled chocolate butter mixture.
Bake in a 9-inch cake pan (preferably a springform or a pan with removable bottom) at 300 F for 45-55 minutes, until a toothpick inserted in the center comes out wet but with a few moist crumbs on the bottom. You can bake in a water bath, but baking time will be around 90 minutes.