Place the chopped chocolate in a mixing bowl. Heat the whipping cream in a small saucepan until it comes just to a simmer. Slowly add the hot cream, stirring and scraping the sides of the bowl until the chocolate has melted. Add the vanilla, the liqueur and continue stirring until the chocolate is smooth with a glossy finish. Add the butter and continue mixing until the butter has melted and the mixture is smooth.
Fill the cups using a small spoon, or pipe the mixture decoratively into the cups.
Note: This wonderful chocolate mixture is called a ganache by pastry chefs and has lots of uses. It can be used as a filling for handmade chocolates or truffles, a filling or topping for layer cakes or cupcakes. Served warm, it can be used as a dessert dip for fruits, cookies or pastries; in can also be used warm as a poured icing on all types of pastries