Melt the chocolate and the butter in a double boiler.
Add the remaining ingredients and cook over medium-low heat, stirring occasionally, until the sugar is completely dissolved and the sauce looks smooth and glossy. (This takes me about 7-10 minutes from the time the chocolate is fully melted.)
Cool slightly, and then transfer to an 8-ounce jar or other airtight container. Store in the refrigerator, where the sauce will keep for quite a few months. It will firm up when cold; reheat gently when ready to serve.