Picking up the right beans for this recipe is critical. You will often see large tins of black beans in the supermarket which are technically turtle beans. The type you'll often find in Mexican dishes. This is not what you want for this recipe.
You need to find fermented black beans. Sometimes referred to as salted black beans. In China, these are often called Douchi or Tochi. You may struggle to find them in your local supermarket, but they are readily available on Amazon or in Asian supermarkets.
You can buy them both vacuum-packed or dried. If you have a choice buy the vacuum-packed variety as you'll have to soak the dried ones for 10-20 minutes before using them. With the vacuum-packed ones, you can just give them a quick rinse first.
When you open a pack of fermented black beans for the first time you may find them a little unappealing. They have a very strong smell, like an extremely potent soy sauce. They also taste very salty and bitter when eaten alone. However, when you add a small amount to your recipe they add a huge amount of savoury flavour and texture!