Hiya Iceywolfie,
So, to be brief: Chocolate is a very poor life choice for a bearded dragon, but is fairly unlikely to cause any serious harm if it's only a little tiny bit.
Unlike in dogs and cats, the problem with chocolate in lizards isn't the necessarily the cocoa, but more likely the dairy. Lizards are unable to metabolize it very well, and it can give them serious digestive issues. Your bearded dragon would probably have the runs and not feel very good for a while after, but of your care was otherwise appropriate, would likely recover. If, however, your dragon was already low-grade ill, such as a slight nutritional difficulty or dehydrated, eating something like that could end up being the "tipping point" that could cause serious issues.
I will be honest, though, I was unable to find an LD50 (the amount in which it would kill 50% of the population, or 1 of every 2 given that amount) for theobromine (the chemical in chocolate that kills dogs and cats) for lizards. While looking for it though, I did find out that there's some evidence to say that our lizard friends don't dream, though, so that's fun!
But let's be not brief at all and do a little thinking on this one, since you're obviously curious and... Well, I am too. We're going to start with some general information on chocolate poisoning in dogs and other animals.
See, while chocolate IS quite poisonous to dogs, the Leader Dogs for the Blind (at least those near me) actually used M&Ms, a popular chocolate candy, as rewards for training their animals in the 80s and 90s. Why? Well, dogs REALLY, REALLY LIKE chocolate, so it's a super good reward for getting them to learn and perform new tricks and tasks. Since there are about 32 M&Ms in an oz of M&Ms, and the dose makes the poison, the number of M&Ms the dogs got in one training session was perfectly safe, if still ill-advised.
See, the toxin compound for animals in chocolate is theobromine, which humans easily and quickly metabolize, but dogs, cats, bears, horses, and other animals struggle to metabolize. As a guideline, humans metabolize it 3-5 times faster than other animals. In dogs, it acts as a stimulant, kind of like certain addictive substances (caffeine or hard drugs). It makes them feel very excited and happy, which is why it's an effective (notice I'm not using the word "good") reward for training. Because of it's drug-like properties, dogs tend to eat more than they can metabolize when they have access to it, causing heart failure due to the stimulate effect, kind of like a drug overdose.
Early symptoms of chocolate poisoning are vomiting, hyperactivity and diarrhea, which may progress to seizures, dangerously high heart rate and death. Sometimes internal bleeding is also reported.
Again, the dose makes the poison. See, since white chocolate doesn't have cocoa, is also doesn't have theobromine, which means it's perfectly safe to give your dog a piece or two of white chocolate. White cocolate still isn't /good/ for your dog, as it has a LOT of fat and sugar, but it won't have the heart-racing drug-life effects on the animal. In milk chocolate, however, there is somewhere around 50-100 mg (usually 60mg) of theobromine per oz, and much more in dark chocolate. Numbers vary greatly, but dark chocolate can have 300 mg/oz or more! The LD50 of theobromine in dogs is about 300 mg/kg, so if a dog ate 1 oz of dark chocolate for every kg it weighed, it would definitely be very sick and have a 50% chance of dying. For reference, cats even are more sensitive to theobromine than dogs, with an LD50 of ~200 mg/kg, but they are less likely to eat chocolate because they cannot taste the sweet of it, which masks the bitter "poison" flavor for dogs. In short, if your dog weighs 20 pounds, it would have a high risk (50% chance) of dying after eating 45 oz of milk chocolate or only 9 oz of dark... Which would be nearly 1,500 regular sized milk-chocolate M&Ms, hence why the leader dogs for the blind used M&Ms as a training treat without any issue since they only gave each dog 10-20 M&Ms a day. I beleive they quite using M&Ms as a reward completely in the 2000s when the toxic effects of chocolate on dogs became more well known, not so much becuase they were putting the dogs in danger, but becuase people would see them using M&Ms for training and do one of two things: 1) get mad at the trainers for poisoning the dogs or 2) disregard the dangers of chocolate because the leader dog people were using it as a treat and then accidentally poison their own dogs by not keeping track of where the chocolate was stored in their house.
With all of this in mind, let's think about our dear beardies... We know that the dairy product in the chocolate is definitely going to be hard for them to digest, but if they are otherwise in good condition, they will probably be fine. But what about the theobromine? Well, let's go with the lowest LD50 I could easily find: The one for cats. If I had to guess, I'd say BDs are actually better at metabolizing theobromine than cats, since BDs are omvivores that eat a LOT of plants while cats are obligate carnivores and theobromine is a plant product, but we'll go with 200mg/kg having an LD50 anyway.
Note, of course, that mammals and reptiles have completely different physiologies and this is just the most basic of guidelines, and that I in no way recommend you feed your lizard any chocolate at all... This is all just theory of the mind bullshit here and I know nothing about chocolate poisoning in reptiles. Anyways...
With an LD50 of 200mg/kg, we need to know a BD's weight. They seem to mostly range between .28 and .5 kg, or 280-500 grams. For the sake of argument, we will again be pessimistic and assume about .3 kg or 300 grams, as a smallish-average sized adult beardie. So, an LD50 of 200mg/kg for theobromine with a .3 kg creature means an LD50 of about 60mg of theobromine. Since milk chocolate has about 60mg per oz, that means your dragon would have a 1 in 2 chance of dying after eating 1 oz of chocolate, or about 32 M&Ms. However, since dark chocolate has 5 times of more the amount of theobromine, the LD50 may be about the amount of chocolate you would find in 6 M&Ms.
To summarize, I think that if your bearded dragon accidentally ate a milk chocolate M&M or a similarly sized piece of chocolate, it would probably be fine. I wouldn't recommend trying the experiment, though.
Hope that satisfies your curiosity!
-Ellen
EDIT: So I did have to correct some of the numbers found in this post earlier. I have dyslexia that is especially bad with numbers, so I swapped a digit (wrote 54 when I meant 45) and then did all my math based on the wrong number. I since have gone back to correct it and have made a few word-choice edits to make this post a touch more clear! Just writing this for the sake of full transparency. Also, if anyone knows the LD50 of theobromine in lizards or other toxic effects of chocolate I haven't considered, I would love to know!
