Re: How do they cook it to be so firm?

The corn is definitely different. I don’t know what they’re called, but I’ve had similar corn in Latin America as well, where the kernels are a bit starchier and chewier.

Korean sweet potatoes are the same as ones labeled “Japanese sweet potatoes” at my local grocery store. Are those what you’re using? They have a dark purple skin and a pale interior, and taste more like chestnuts. American sweet potatoes are now common in Korea also and they’re called “squash sweet potatoes” because they taste more like winter squash.