Disaronno Amaretto
On the back of the bottle it says try it over ice with a squeeze of lemon. Don't. The lemon just did nothing apart from hide trying to hide the taste. Just add a few more ice cubes which seems to cut the thickness and test just right.
Disaronno web site
In Portugal you need to ask for "amarguinha". Not as dark and thick as Disaronno but just as nice. Over ice!