Start by bringing a medium pot of water to a rolling boil, then carefully add your shelled pistachios. Let them cook for around 3 minutes. Drain thoroughly, then transfer the pistachios onto a clean kitchen towel. Fold the towel over the pistachios and rub vigorously to loosen and remove the skins. Set the pistachios aside, discarding the skins.

In a small microwave-safe bowl, combine the unsalted butter and 1/4 cup of the milk or cream. Heat the mixture in 20-second intervals until the butter melts and the milk is warmed. Stir in the chopped white chocolate until it melts completely, adding another 10-15 seconds of heat if necessary. Alternatively, you can melt these ingredients together gently over low heat in a small saucepan.

Add the skinned pistachios to a blender. Pour in the remaining 1/2 cup of milk or cream, along with the powdered sugar and kosher salt. Scrape in the melted white chocolate and butter mixture. Pulse the blender on medium speed, stopping occasionally to scrape down the sides, until the pistachios form a smooth paste. Increase the blender to high speed for 30 seconds to 1 minute to achieve a creamy consistency.

Once the pistachio cream is velvety and smooth, transfer it to an airtight container. Refrigerate and use within two weeks for the best flavor.