The heat doesn't come from chili at all — it comes from chopped pieces of a pepper vine's stem and root, and locals rate the root so highly that it sells for more than the stem.
Chunks of a climbing pepper vine called chui — its peeled stem and root, not its fruit — are simmered for hours with beef or mutton until they turn tender and release a sharp, horseradish-like bite into the gravy. Khulna's markets and eateries treat it as the region's defining curry, distinct from anything found elsewhere in Bangladesh.