Use the following proportions for ganache, based on 8 ounces/227 grams chocolate:

If you are adding butter, use high fat butter, preferably cultured, for the best flavor and texture. I like to add, in addition to the cream, up to one-third of the weight of the chocolate in butter and up to ½ teaspoon of liqueur per ounce of chocolate. So, for 6 ounces/170 grams chocolate, I add 1 to 2 ounces/28 to 56 grams (2 to 4 tablespoons) butter and 1 tablespoon/15 ml liqueur.

Ganache takes from 3 to 7 hours to cool, depending on the quantity and the temperature of the room. It should be left, uncovered, for about an hour to allow the heat to escape. Then it should be covered to prevent evaporation and left undisturbed to set. It is fine to stir gently two or three times during the first 30 minutes while the temperature is still above 85°F/29°C to equalize the temperature. (Rapid cooling would not give the cocoa butter a chance to form small stable crystals that melt gradually when eaten.) When the ganache is firm enough, press a piece of plastic wrap directly on the entire surface to keep the edges from drying.