These are quite popular and I think they are one of the older sweets. My guess is they were borrowed cuisine from China. I've mentioned before that Japan got its baking culture largely from Europe. These steam cakes are not made in an oven, but usually on the stove top. That would explain their long history and the nostalgic popularity of them. If you'd like to try your hand at making your own (sans bizarre-looking beans), there's a recipe here.