Preheat oven to 325 degrees F. Grease an 8 X 2-inch round pan and line the bottom with parchment paper. Have all ingredients at room temperature.
Combine the chocolate and butter in a double boiler, heat and stir often until the chocolate and butter are warm, melted and smooth. Remove from heat and whisk in the egg yolks.
With a mixer fitted with the whisk attachment beat the egg whites and cream of tartar on medium speed until soft peaks form. Gradually add the sugar and beat until the peaks are stiff, but not dry. Fold the egg whites into the chocolate mixture a quarter at a time.
Scrape the batter into the prepared pan. Place the pan into a shallow baking dish. Fill the baking dish with hot water until it reaches halfway up the outside of the cake pan. Be careful to make sure the water does not get into the cake batter.
Bake the cake for exactly 30 minutes; the top will have a thin crust and the interior will still be gooey.
Set the pan on a cooling rack and cool completely. Refrigerate overnight.
To unmold, slide a knife around the cake to detach it from the sides of the pan. Invert the pan and remove the parchment paper. Turnover onto serving dish.
Top with Sea Salt Caramel Sauce and garnish with sea salt or pecans, if desired.