(Nana and I are traveling until July 7, but we've put a couple posts in the pipeline to tide you over until then.)
Beni-imo (紅芋) is an Okinawan variety of sweet potato. It's actually deep purple inside, and very sweet. What you see above is an advertisement for the classic Okinawan souvenir: half-shell cookies filled with a sweet paste made from beni-imo. You can also find beni-imo ice cream and these beni-imo fritters.
We also found some beni-imo sticks fried in black sesame batter.
Apparently, beni-imo is a fairly sensitive variety: there are strict bans on importing satsuma-imo strains of sweet potato from Kyushu, with signs posted throughout the airport warning against the dire consequences of any illicit inter-sub-species interaction. (These are not to be confused with the signs warning kids against picking up old WWII grenades, which kind of look like potatoes. More on that later.)