In a saucepan over medium heat, bring the cream to the boiling point and remove from heat.
In a heat-proof bowl, pour the hot cream over the chocolate, then gently blend with a wooden spoon or whisk until the chocolate is melted and perfectly smooth. Blend in the liqueur and vanilla.
Let cool for a few hours, or until it reaches the consistency of frosting.
Notes
You can replace the flavoring with more heavy cream.
To thin out the ganache, add 2 to 3 teaspoons of tepid water or cream.
For glazing a cake, let come almost to room temperature, or barely lukewarm. If left to completely cool, it will be difficult to use as glaze. If too thick and unpourable, thin down as above or gently reheat it. To glaze, put the cake on a wire rack set on a rimmed baking sheet, pour the ganache over the cake, smooth the top quickly with a wide frosting spatula and give the rack a tap.
For fluffier ganache, let cool to room temperature, whip on high, taking care not to overwhip it into a grainy consistency, then use immediately before it firms back up and becomes unspreadable.
Storage
Refrigerator - Up to 1 week.
Freezer - Up to 1 month, double wrapped with a film of plastic placed directly on the surface of the ganache, and again to cover the container.
Allow to come to room temperature before using (about 3 hours from the fridge or overnight from the freezer).