Heat together chocolate and water in small saucepan over low heat until chocolate is melted.
Stir in sugar until smooth. Bring to boiling; boil for 2 minutes, stirring constantly.
Remove saucepan from heat. Stir in butter, liqueur and vanilla until butter is melted. Serve warm or at room temperature. Sauce will thicken on standing. Makes 1 cup.
The sauce can be made several weeks ahead and refrigerated, covered. To reheat, pour the sauce into a small saucepan and gently reheat over low heat until sauce is smooth.
