This gelato has become a signature dish from Modernist Cuisine; we serve it at most of our events. Vegans particularly enjoy it because it is as smooth and intense as the best gelato, but contains no eggs or dairy.
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Measure the cold water into a pot. Combine the sugar, tapioca starch, salt, and xanthan gum. Dry-blend the thickening agents with a whisk to help ensure that they are distributed evenly in the next step. Blend the dry ingreidnets into the water by using an immersion blender. Bring the mixture to a boil while blending constantly.
Remove the pot from the heat. Blend the pistachio butter and pistachio oil gradually into the pot until thoroughly combined using an immersion blender. Chill the mixture over an ice bath. Whisk the mixture occasionally to help it cool evenly. Transfer the mixture to an ice-cream machine, and churn it until it thickens and freezes. Transfer the gelato to a container, press plastic wrap onto the surface of the gelato, and seal the container. Then place it in the frezzer until the gelato is firm, at least 4 hours. The plastic wrap prevents a skin from forming on the gelato. You may need to temper the gelato at room temperature slightly before serving it.