Bring a medium pot of water to a boil, add the shelled pistachios, and boil them for 3 minutes before draining them onto a clean kitchen towel. The nuts should look slightly loosened at the surface, which helps the skins slip off more easily.

Fold the towel over the hot pistachios and rub them firmly to release as much skin as possible, then discard the skins and let the nuts dry completely before adding them to a blender. The cleaner and drier the nuts are, the greener and smoother the finished cream will be.

Place the butter and 1/4 cup of the milk in a small microwave-safe bowl and heat in 20-second bursts until the butter melts and the milk is hot, then stir in the chopped white chocolate until smooth, heating again for 10 to 15 seconds if needed. The mixture should look glossy and fluid, with no unmelted chocolate bits left behind.

Add the dried pistachios, confectioner's sugar, and salt to the blender, then scrape in the warm white chocolate mixture and pulse on medium speed until a thick paste forms. It will first look crumbly, then clumpy, then turn into a dense green paste.

Pour in the remaining milk a few tablespoons at a time until the mixture reaches the thickness you want. Stop before it looks too loose, because the cream firms up noticeably once chilled.

Scrape down the blender several times and finish blending on high for 30 seconds to 1 minute, then transfer the cream to an airtight container and refrigerate. It is ready when it looks smooth and creamy and a tiny smear rubbed between your fingers no longer feels gritty.