In a food processor, blend the sugar until it is finely ground.

Add the pistachios to the food processor and blend until they are finely ground and have a paste-like consistency.

Melt the white chocolate chips in a double boiler or in the microwave in 30-second intervals, stirring until smooth.

In a saucepan, combine the ground pistachio mixture, melted white chocolate, milk, and butter. Cook over low heat, stirring constantly, until the mixture is well combined and has thickened slightly, about 10 minutes.

Transfer the pistachio cream to a bowl and let it cool to room temperature. Once cooled, cover and refrigerate until it is set, about 1 hour.