Cover the cashews and dates with boiling water for 15 minutes. Drain, discarding the water, and transfer to a high-powered blender (see notes). Blend with around half of the cold water until completely smooth and creamy. Add in the rest of the water, the salt, vanilla, cacao and sweet potato purée and blend again until smooth.
Churn the mixture in an ice cream maker until thickened and scoopable (this took about 40 minutes for my ice cream maker). Transfer to a loaf pan and smooth out then freeze for at least 4 hours until solid.
If you want to decorate it: stir together the coconut oil and cacao powder. Drizzle it over the frozen ice cream and quickly scatter on the pistachios then return it to the freezer.
When ready to serve, let it sit at room temperature for 5-10 minutes to soften before scooping