In a medium saucepan, combine the milk and heavy cream. Heat over medium heat until just simmering, stirring occasionally.
In a separate bowl, whisk together the sugar, cornstarch, and salt. Add the egg yolks and whisk until smooth.
Once the milk mixture is simmering, slowly pour it into the egg mixture while whisking constantly to temper the eggs.
Return the mixture to the saucepan and cook over medium heat, stirring constantly, until it thickens and begins to bubble, about 2-3 minutes.
Remove from heat and stir in the pistachio paste and butter until fully combined.
Transfer the pistachio cream to a bowl, cover with plastic wrap directly on the surface to prevent a skin from forming, and refrigerate for at least 2 hours or until chilled and set.
Serve the pistachio cream in dessert cups, on pastries, or as a filling for cakes.