Combine tapioca starch, sugar, water, and heavy cream in a saucepan. Mix until well combined.
Place the saucepan over medium heat and simmer the mixture for 8-10 minutes, stirring constantly. When it slightly thickens and coats the back of a spoon, remove it from the heat.
In a large bowl, mash the avocados or blend them until smooth or slightly chunky.
Combine the cream mixture and avocados, mixing until well combined.
Transfer the mixture to a container and refrigerate for at least 4 hours or until completely chilled.
Churn the chilled mixture in an ice cream maker according to the manufacturer's instructions until it reaches a soft-serve consistency. Add the pistachios gradually toward the end.
Transfer the ice cream to a freezer-safe container. Place a piece of plastic wrap directly on the surface, then cover it completely to prevent ice crystals from forming. Store it in the coldest part of your freezer and freeze for at least 4 hours or until firm.
Let it sit at room temperature for a few minutes to soften slightly before scooping. Serve and enjoy!
Notes
Avocados: Use ripe avocados, preferably the Hass variety, for a creamy texture and rich flavor.
Tapioca starch: This stabilizer thickens the mixture and prevents ice crystals from forming by absorbing excess water. Alternatively, you can use cassava or arrowroot flour in a 1:1 ratio. Another option is to use slightly less cornstarch or a small amount of xanthan gum.
Heavy cream: Higher fat content of at least 36% helps create a creamy and smooth texture. Full-fat coconut milk or coconut cream can be used as substitutes, while half-and-half may yield a less creamy result.