1/2 vanilla bean or 1/2 teaspoon pure vanilla extract
Mix the water and sugar in a large heavy-bottomed saucepan. Cover and cook over low heat until the sugar dissolves. Increase the heat and boil uncovered until the sugar turns a medium brown, about 5 to 7 minutes, stirring occasionally with a wooden spoon. Watch it carefully at the end, as it will go from caramel to burnt very quickly. Stand back to avoid splattering, and gradually add the cream and the seeds scraped from the vanilla bean (or vanilla extract). Simmer until the caramel dissolves and the sauce is smooth and thick, about 2 minutes. Serve warm, or add another 1/4 cup of heavy cream and serve room temperature.