Because Aji peppers are difficult to find in the states, we normally use a combination of Serrano and jalapeño peppers for this recipe. In case you are still interested in Aji peppers, here is a bit of information I was able to gather:
Aji peppers are a group of chili peppers native to South America, especially popular in Peruvian and other Latin American cuisines. "Ají" (pronounced ah-HEE) is simply the word for "chili pepper" in many Spanish-speaking countries, but it typically refers to specific varieties with distinct flavors and heat levels.