Until 2014, when TWO completely independent ice cream stores opened.
1.) Just north of the airport in the town of Koseda, Cafe Mori offers soft-serve vanilla made from Jersey milk.
2.)Near the town of Mugyo, right next to Pension Echoes and Mori-no-Ferry Cottages, Sola Umi spoons out a variety of gelatos and sorbets.
At Solaumo, I always get a double cup so I can pick two flavors -- something exotic like papaya or avocado(?!) or my favorite coconut, and something more traditional like chocolate. A small double-cup costs a little less than 500 yen. They also serve coffee.
I advise you call ahead though, because they keep some unusual business hours.
Yakushima Gelato SolaUmi
Location: Mugio (30.262792,130.6045482)
Business Hours: 1pm to 5:15pm, closed on Tuesdays, Thursdays, and the first Saturday of the month*
*Hours of business may change, so please check the website or call ahead.