Melt the chocolate in the top of a double boiler (or a heat-proof bowl set over a pot of simmering water); add the sugar, butter and cocoa. Simmer 15 to 20 minutes, stirring often until the sugar has dissolved into the melted butter and chocolate. Add the cream and cook 10 minutes longer, stirring constantly, and then slowly stir in the vanilla. NOTE: the mixture will look really grainy just before the heavy cream is added. It comes together as soon as the heavy cream is stirred in.
Remove from heat and let cool. This can be kept in the refrigerator for several weeks.
Notes
*If you are preparing this recipe as gluten-free, just be sure to use brands of chocolate and vanilla that are known to be GF.
*When the cream is added, if it has the appearance of curdling- do not be alarmed! It will eventually become smooth.