Preheat oven to 325°F.
Using 1/2 tsp butter, grease a 9 inch springform pan and line bottom with a parchment round. Cover pan underneath and along sides with foil and set in a roasting pan. Bring medium saucepan of water to boil.
Combine the chocolate, butter, and Kahlua in a metal bowl set over simmering water or in the top of a double boiler. Melt the mixture, stirring constantly, until smooth and creamy, about 5 minutes, reserve.
Meanwhile, combine eggs, vanilla, sugar and salt in a large bowl and whisk with an electric mixer until frothy and almost doubled in volume, about 5 - 10 minutes. Fold 1/3 of egg mixture into the chocolate mixture using a rubber spatula. Repeat this procedure 2 more times, until all of the egg mixture has been added into chocolate mixture.
Pour batter into prepared springform pan and place in the roasting pan. Pour enough boiling water into the roasting pan to come about halfway up the sides. Bake until cake has risen slightly and edges are just beginning to set, about 40-45 minutes. Remove cake from roasting pan and cool on wire rack to room temperature Remove foil, cover and refrigerate overnight.
Remove cake from refrigerator about 30 minutes before serving. Garnish with confectioners sugar or cocoa immediately before serving.