This is a little more creativity to get the desired effects. Ultimately our joker is using a nerve agent or 'nuerotoxin' of some type. They operate by interfering with the brains ability to communicate with a muscle (can go further here, it's interfering with the sodium component of your muscle, causing it to mass contract). Most common form of death here is actually asphyxia...your diaphragm that controls breathing is ultimately a muscle and not being able to communicate with the muscle that lets you breathe is a quicker death. This is followed shortly by heart failure/arrhythmia (heart also being a muscle).
So getting something that can mess with your muscle control that kills through the same means is actually quite plausible...if you can't control your facial muscles, you're not going to be able to control your diaphragm and death from asphyxia comes shortly after.
Strychnine was the first path I followed...it's known to cause the same effects risus sardonicus has as described by HDE, but this is oral and a pretty high dosage to get these effects. Sarin and cyclosarin (Iraq/Iran during their conflict) is another possible one here, though I doubt the joker would want to watch his victim smile for days on end before perishing. VX is another line of thought here...never really used and far deadlier than Sarin.
As an alternative...Batrachotoxin. This comes from a frog and is used by certain tribes as a blow dart poison. Same effect as above, though this is known to target the heart a bit faster (15-20 mins) and works in tiny (microgram) levels. A little modification, and perhaps the facial controls can go first. There is around 100 variations of these toxins (including tetrodotoxin...that poison blowfish are well known for) and it's feasible that atleast one of these toxins will have a pronounced affect on ones face causing the painful grin.
The majority of these toxins appear to be water born...getting them into an aerosol form seems feasible as well (kinda a misty gas).
This does leave the potential of a twisted smile on their face (caused more by all muscles in their face contracting) while dying, however the maniacally laughing trait is a bit of a miss on this route.
So option 2.
Contrary to popular belief, laughing gas does not make one laugh...a few popular movies and tv shows have tried to show that laughing gas makes on laugh at all costs, but it's not really true. It will make you high enough that more things seem funny, yes...but it's not forced laughter in the degree our joker wants here. The best I can find for inducing laughter is actually Psilocybin, the active ingredient in psychedelic mushrooms. These will cause a person to laugh uncontrollably for no real reason (along with quite a few other affects), however the experience seems subjective and everyone has a different response. Combining this with another toxin that does the actual killing might ****might**** result in seeing someone laugh themselves to death (1 in 1000?), but it's definitely not a guaranteed outcome.
Ya, painfully contorted smile caused by painfully contracting facial muscles seems to be the only route. No laughing yourself to death I guess.