This is a very special cactus to me. I got it at the Eastern Market in Detroit in 2013. Since then, I haven't found the name of this cactus. It is 3 inches in diameter and 2 inches high. I have no idea how to identify this cactus. I tried looking online to see what kind of cactus this is, but some cactuses that looked completely different had the same name. Will someone please help with identifying my cactus? Thank you.

Henry, age 7