Heat the cream, in a small saucepan until it is hot and just simmering around the edges. Do not boil.
Place chocolate in a medium bowl and pour over the hot cream. Let stand for 5 minutes. Add the remaining ingredients, if using, and whisk until the chocolate has melted completely and the mixture is smooth.
Cover with plastic wrap and set aside to cool and thicken. It will fully cool within 2 hours at room temperature. You can refrigerate if you are in a hurry, but the frosting may not set evenly. Stir it a few times as it sets in the refrigerator so it remains even and smooth.
If you wish to use it as a glaze, then cool only enough to thicken but still pourable. Pour over cake to glaze.
For a frosting, whip the ganache on medium speed until light and fluffy, about 3 minutes. It should form stiff peaks.
Makes about 2 cups of frosting (enough for 12 cupcakes) or lightly frost a 2 layer cake.