How to Make It
Preheat the oven to 325°F. Using the 1/2 tablespoon butter, grease a 9-inch springform pan. Line the bottom of the pan with a parchment round. Tightly cover the pan underneath and along the outer sides with foil and set in a roasting pan. Bring a medium saucepan of water to boil.
Combine the butter, chocolate and Kahlúa in a metal bowl set over simmering water or in the top of a double boiler. Melt the chocolate, stirring constantly, until smooth and creamy, about 5 minutes; reserve.
Meanwhile, combine the eggs, sugar, vanilla and salt in a large bowl and beat with an electric mixer until frothy and almost doubled in volume, about 5 minutes. Fold 1/3 of the egg mixture into the chocolate mixture with a rubber spatula. Repeat this process 2 more times until all of the egg mixture has been folded into the chocolate.
Pour the batter into the prepared springform pan and add enough boiling water to the roasting pan to come halfway up the side of the springform. Bake until the cake has risen slightly and the edges are just beginning to set, about 25 minutes. Remove the cake from the roasting pan and cool on a wire rack to room temperature. Remove the foil, cover and refrigerate overnight.
Remove the cake from the refrigerator about 30 minutes before serving. Remove the springform side, invert the cake onto a large plate, and peel away the parchment paper from the bottom. Invert the cake onto another large plate or serving platter and garnish with confectioners' sugar or cocoa powder immediately before serving.