This reminds me of a similar post several years back. Butterscotch syrup (or ice cream topping) was mentioned. Both of those can have cream, butter and or milk solids in them as well as any number of other ingredients, putting fat content into the beer didn't seem to make sense, or milk solids (which would have protein as well as lactose). So, it didn't make much sense to pour that into the beer like you would a Lyle's Golden syrup that is common procedure in Britain. Makes you think of the brewers who use Jolly Ranchers for their Watermelon ales though!
How about some Werther's Originals?
Or just brew a very malty, caramel bitter or something on the lines of a milk stout (with some lactose added) and use the Ringwood yeast???? It's certainly known to produce diacetyl. Hard to tell, having never tasted what you are describing. It would be unique in any event.
P.S. Got sidetracked mid post and noticed the new posts! I've added chocolate extract to a Young's Double Chocolate Stout clone -- You can do that right at bottling if you want and taste and add until you like what you taste. It will soften a bit with conditioning and bottle age. I don't really secondary, but I do rack a day or two before I bottle. I added the chocolate extract then, and tasted. It softened a bit even in the next few days before I bottled. Then, more afterwards. There will be differences in flavor between brands of flavoring as well. That's another place you would need to experiment or go by the trial and error method. And, you know, those extracts probably have diacetyl added right to them! (Read your bottle label) Just like inexpensive microwave popcorn... so beware...