Put potatoes in a large pot and add enough cold water to barely cover. Stir in 1 teaspoon salt. Bring to a boil over high heat, covered. Reduce heat to medium and boil gently until potatoes are tender when pierced, about 15 minutes. Drain; return to pot, off the heat.
In a medium bowl, combine Cashewmilk Yogurt, 1/2 teaspoon salt, and 1/3 cup each parsley, dill, and chives. Set aside.
Mash hot potatoes with a potato masher until all large chunks are broken and potatoes are fluffy. Add Cashewmilk, 2 tablespoons butter, 1/2 teaspoon salt, and the pepper; mash to blend. Stir herbed yogurt into potatoes. Add salt and pepper to taste. Transfer to a serving bowl, top with remaining 1 tablespoon butter, and scatter 2 tablespoons mixed chopped herbs over the top.
To make ahead: You may keep the thoroughly mashed potatoes for up to one day, chilled before adding the herbed Cashewmilk Yogurt mixture. Reheat potatoes in a microwave until hot (if they’ve stiffened up, add additional Cashewmilk 1 tablespoon at a time, until creamy). Stir herbed Cashewmilk Yogurt into warmed potatoes and proceed as directed.