In a small heavy saucepan combine water and sugar and boil, stirring, until sugar is dissolved. Remove pan from heat and whisking cocoa powder, whisking until smooth. Whisk in salt, cream, and butter and return pan to moderately low heat, whisking until butter is melted. Simmer sauce until thickened slightly, about 2 minutes, and stir in vanilla and Sambuca. Cool sauce completely and transfer to a jar with a tight-fitting lid. Sauce keeps, covered and chilled, 1 month. Serve sauce warm over ice cream.