Blanch the pistachios Boil a medium pot of water.Add shelled pistachios and boil for 3 minutes. Drain and transfer to a clean kitchen towel. Rub pistachios in the towel to loosen the skins, then discard skins. Be sure to remove all the brown bits to reduce grain.
Melt the Chocolate MixtureIn a small microwave-safe bowl, add butter and ¼ cup milk. Heat in 20-second bursts until butter is melted and milk is hot. Stir in white chocolate until melted. If needed, heat for 10-15 more seconds. (Can also be done on the stovetop over low heat.)
Blend Everything TogetherAdd blanched pistachios to a blender. Pour in the remaining ½ cup milk, powdered sugar, and salt. Scrape in the melted white chocolate mixture. Pulse 3-4 times, stopping to scrape down the sides. Blend on high power until smooth and creamy.
Store & UseTransfer to an airtight container and refrigerate. Keeps fresh for up to 2 weeks in the fridge.