Fill a medium sized saucepan halfway with water and bring to a low boil. Add the shelled 1 1/2 cups unsalted pistachios, stirring, and boiling for 4 minutes.
Drain in a colander and then spread in an even layer on a tea towel or paper towels.
Cover and roll them around to loosen the papery skins. Discard as much of the skins as possible. It will not impact flavor, but skins left behind or using any brown nuts will leave flecks in your cream sauce. Place the pistachios into a high powered blender or food processor along with 1/2 cup of the heavy cream. Pulse until a paste starts to form. Set aside.
In a small saucepan, heat the 3 tablespoons unsalted butter and 1/4 cup of the cream over low heat. When it starts to steam, stir in the 3/4 cup white chocolate chips until smooth.
Transfer the chocolate mixture along with the the 3 tablespoons powdered sugar and 1/4 teaspoon coarse sea salt, to the blender. Blend for 1 minute, scraping down the sides as you go, until smooth and creamy.
Transfer to an airtight container and chill for at least 1 hour before using.
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Notes
We found that tea towels work best for removing the skins. This process is not necessary, but there will be flecks in the cream if you don't get them off. It is tedious, plan for about 10-15 minutes.