Combine mashed ube, sugar, water, cream, and vanilla in a medium pot. Blend or whisk until smooth. Whisk in the tapioca starch before turning on the heat. Add ube extract, if using.
Cook over medium heat for 8–10 minutes, stirring constantly, until slightly thickened and coating the back of a spoon.
Remove from the heat and let cool for about 15 minutes. Transfer to a container, then cover. Refrigerate for at least 4 hours, until completely cold.
Churn according to your ice cream maker’s instructions until thick like soft serve.
Transfer to a freezer-safe container. Press plastic wrap onto the surface, then cover. Freeze for at least 4 hours, until firm. Let soften briefly before scooping if needed.
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Notes
Ube: Use cooked, mashed ube or purple sweet potatoes. If using sweetened ube halaya, reduce the sugar to taste.
Ube extract: Optional, for more flavor and purple color. Add a few drops at a time.
Tapioca starch: Also called tapioca flour or cassava starch. Thickens the base and helps reduce iciness. Don’t use cassava flour.
Heavy cream: Full-fat coconut milk or cream also works, but changes the flavor and texture. Half-and-half makes it less creamy.