Mochi ice cream is a ball of ice cream wrapped in a thin layer of mochi - a dough made from glutinous rice that is steamed until it turns soft, stretchy, and slightly sweet. You eat it in two or three bites. The outside is cool and pliable, the inside is cold and creamy.
The idea has two parents. The older one is daifuku, a traditional Japanese confection of mochi wrapped around a sweet filling, usually red bean paste. The newer one is Yukimi Daifuku, the Japanese convenience-store classic that put vanilla ice cream inside mochi in the early 1980s. The version most Americans know - sold from freezer cases in grocery stores - was popularized in Los Angeles in the 1990s and travels frozen, sometimes for months.
That last part matters. Mochi changes fast after it is made. Within hours the exterior begins to firm; frozen and thawed, it turns thick and chewy in a way fresh mochi never is.
The dough - sweet rice flour, sugar, a little cornstarch, whole milk and a touch of oil - is made in our Harvard Square kitchen, and every piece is wrapped by hand around the ice cream when you order it. The skin stays thin and yielding, the seal is pressed by hand, and no two pieces come out quite the same.
If you are trying mochi ice cream for the first time, start with matcha or black sesame, two of the three flavors that never change, and eat it soon after it is wrapped.