Step 1: Blanch the pistachios. Bring a small pot of water to a boil, add the shelled pistachios, and boil for 1-2 minutes. Drain them and immediately rinse under cold water.
Step 2: Remove the skins. While still warm, gently rub the pistachios between your fingers or in a clean kitchen towel to remove their skins. This step is crucial for achieving a vibrant green color and smooth texture.
Step 3: Grind the pistachios. Place the blanched and skinned pistachios in a food processor. Process until a fine, crumbly flour forms, then continue processing until a thick, somewhat oily paste begins to form (this may take 5-7 minutes, scraping down the sides as needed).
Step 4: Melt the white chocolate. In a heatproof bowl set over a saucepan of simmering water (bain-marie) or using a microwave in 30-second intervals, gently melt the chopped white chocolate until smooth.
Step 5: Combine ingredients. In a small saucepan, melt the unsalted butter over medium-low heat. Add 1/2 cup of the whole milk or cream and bring it to a gentle simmer.
Step 6: Finish the cream. Add the pistachio paste, melted white chocolate, confectioner's sugar, and kosher salt to the saucepan with the milk mixture. Stir continuously until everything is well combined and smooth. If the mixture is too thick, gradually add the remaining 1/4 cup of milk or cream until you reach your desired consistency.
Step 7: Cool and store. Transfer the pistachio cream to a clean, airtight jar. Allow it to cool completely to room temperature before refrigerating. The cream will thicken further as it chills.