CADECA (Casa de Cambio) is the official government exchange house in Cuba where you can exchange most currencies into Cuban Convertible Pesos (CUC). There are branches of the CADECA in Cuba at the airport, Havana, Viñales, Varadero, Cienfuegos, Trinidad, Camaguey, Santiago, Baracoa and in most populated areas. However, I have come across a couple of CADECA’s in Cuba that only exchange between the 2 local currencies and do not have the facility to exchange foreign currencies, so if unsure, check with the security guard at the door before joining the queue!
The exchange rates at the CADECA and at the bank are set daily by the Cuban government, so I have found that there is very little difference between the exchange rates found at either the CADECA or the bank in Cuba. Therefore, there is no real point in spending the day shopping around for the best rate. I tend to use the CADECA more, as for me it is fast and fairly pain free and more conveniently located. However, make sure you get a receipt for each transaction, take your time before leaving the teller and check that the teller has given you the correct amount back, as they count the money really fast in front of you and I have been short changed in the past!
The CADECA in Cuba can also give you a cash advance on your credit card (credit cards accepted in Cuba). They usually charge a one-off transaction fee, you will then need to speak to your credit card company to see what interest charges they will apply on top of the transaction fee. One way to avoid accumulating interest charges is to put money onto your credit card, this way you can use it as a debit card.