Place rack in center of oven. Preheat oven to 350 degrees. Lightly butter 9” round cake pan, line with parchment paper, butter paper. Put cake pan in a shallow roasting pan and set aside.
Put chopped chocolate in medium bowl. In a 2 quart saucepan, mix 1 cup of sugar and bourbon and cook over medium heat, stirring occasionally until sugar dissolves and comes to a full boil. Pour over chocolate, stir with whisk (rubber spatula) until mixture is smooth. Piece by piece, stir butter into chocolate mixture, making certain each piece is melted before you add another.
Put eggs and remaining ⅓ cup sugar in medium bowl and whisk until eggs thicken slightly. Beating with whisk, add eggs to the chocolate mixture and whisk until well blended. Gently whisk in flour.
Pour and scrape into pan. Smooth with spatula. Boil water first and pour into jelly roll pan 1” around cake pan.
Bake 30 to 35 minutes, until a thin, dry crust forms. Remove from water bath, wipe pan dry, put plastic or parchment over top. Invert onto flat platter, pull off parchment and quickly but gently invert again.
Keep at room temperature 1 day, or refrigerated 3 days.
Serve with whipping cream, whipped with a little sugar, vanilla and bourbon.