I have a joint US and EU citizenship and I live in the US.?
The only way you can buy a flight ticket to Cuba from the US is through one of a few charter companies authorized by the US government to operate flights to the island. These companies are only authorized to sell tickets to two kinds of people: Cubans and US residents or citizens traveling to the island under one of twelve categories that the government has set aa valid reasons to go there. The US constitution forbids doing tourism in Cuba through it's economic embargo of the island (it's actually the US Treasury that would have an issue with you going there as you would be spending US dollars there). So, no, you cannot travel directly to Cuba from the US with a EU passport. You cannot even buy your tickets to Cuba from a third country directly on the internet or by calling a US-based travel agency or the US-based customer service office of an international airline. Depending on where you live, you're better off contacting a Toronto or Montreal based travel agency or a Bahamian travel agency.