Coffee (espresso) Beans
The caffeine amount of coffee beans can vary depending on bean size and how darkly they were roasted.
This amount is the average caffeine content of an average size arabica bean which is the most common coffee bean used in making coffee or chocolate covered coffee beans.
In general arabica coffee beans have around 1.1% - 1.4% caffeine per bean (w/w), but this isn't a true picture of all of the caffeine they contain. Green coffee beans are lower (0.6% - 1.1% caffeine)
There Is More Caffeine in a Bean Than What Is Extracted
1.1% to 1.4% represents the amount in the first water extraction of a coffee bean which would be about 1-2 mg depending on the exact size of the bean.
However, there is still caffeine present in a coffee bean after the first extraction. We know this because used coffee grounds still contain caffeine.
2013 research deteremined that the majority of coffee beans had 10.0-12.0 mg caffeine per gram weight. One sample had 19.9 mg/g.
There's about 7 beans per gram of coffee - so this gives a value of around 1.4 - 2.8 mg caffeine per bean (somewhat lower than other).
Anyone that as ever consumed chocolate covered espresso beans can attest that they pack a much bigger punch of caffeine than 1-2 mg per bean.
One manufacturer (Crackheads) had their dark chocolate covered coffee beans tested for total caffeine and found that they were about 7 mg per bean including the chocolate. This seems quite high (the dark chocolate accounts for some of the extra caffeine).
There is no difference between a standard coffee bean and what is titled espresso beans. This is a title given to a blend by a coffee roaster that is intended for use in an espresso machine. You could grind the beans and use in a filter coffee machine - it is the brewing method that will govern how much caffeine ends up in your cup.
Robusta Beans (i.e. fruit from the Caffea Robusta species) will have more caffeine than arabica beans (i.e. fruit from the Caffea Arabica species) and it is generally accepted that they contain double the caffeine of arabica beans.
Therefore each robusta bean could contain around 5-10 mg of caffeine. In recent years better tasting Robust beans have been developed - leading to a new trend of super highly caffeinated coffees.