Whisk tapioca starch, sugar, water, and heavy cream in a medium pot until smooth.
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.
Mash or blend the avocados, then stir into the cooled cream mixture. Taste and add more sugar if needed, stirring until dissolved.
Cover and refrigerate for at least 4 hours, until completely cold.
Churn according to your ice cream maker’s instructions until thick like soft serve. Add the pistachios during the last few minutes.
Transfer to a freezer-safe container. Press plastic wrap directly onto the surface, then cover. Freeze for at least 4 hours, until firm. Let it soften for a few minutes before scooping if needed.
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Notes
Avocados: Use ripe avocados that feel slightly soft when squeezed. Hass works well.
Tapioca starch: Also called tapioca flour or cassava starch. Thickens the base and makes ice cream less icy.
Heavy cream: Full-fat coconut milk or coconut cream also works, but changes the flavor and texture. Half-and-half makes it less creamy.
Sugar: Use granulated sugar. Adjust to taste after adding the avocados.
Pistachios: Chop into small pieces. Substitute almonds, chocolate chips, or coconut flakes, or leave out.