it could be the fat content
cats love fat, it being present in all their meaty meals. Cats famously love milk and cream, which could also be because of the fat content. Standard chocolate (e.g. cadburys and equivilants) has a hell of a lot of fat in it, so it could be this they're going for rather than the sugar.
( ferret -> twitter.com/syfyautocorrect <-, Sat 24 Dec 2011, 3:35, 2 replies)

My old cat used to go nuts for CHOCOLATE CAKE!
Proximity to it turned him into a cake-eating zombie, who would just blindly walk/climb/jump towards the cake, unrelentingly.

If I had a plate with a cake on it, and held the plate in the air so he couldn't get at it, he'd attempt to climb up me.

The only course of action was to attempt to eat ALL the cake in one go, but then he'd just try and climb into my face to access the cake I was chewing.

Finally, you'd have to dispose of the packaging in a proper dustbin, because he'd knock over any kitchen bin or waste paper basket to eat the wrapper.
(eViLegion Chief Commissioner of the Scottish Lunacy Board, Sat 24 Dec 2011, 12:49, closed)

Yeah, I'd go for lipid and protein content.
Chocolate always has some kind of lipid and cakes and the like have protein in the shape of egg whites and often have lipids also.
As a kid we had a cat who would beg for peanuts and crisps and I put that down to the salt and fat. Oh, yes, another thing cats love is salt and I'm sure I read their food is much saltier than dog food which is why you can feed a cat dog food (plus other stuff for nutrients) but shouldn't feed a dog cat food.
Oh, as for Rats I think they're evolving so that they can eat almost anything including (relatively) large amounts of cyanide and arsenic.
(Me, I'm not... in any way intoxicated, Sat 24 Dec 2011, 16:50, closed)