In a small heavy saucepan, combine the cream and brown sugar. Bring the mixture to a boil over mod-high heat, whisking occasionally, and boil it, whisking, until sugar is dissolved. Remove pan from heat and add chocolates, whisking, until they are melted. Whisk in the butter and the Amaretto. Continue to whisk until the sauce is smooth. Let cool slightly.
Makes about 3 cups.