Prepare Crumbs: Pulse biscuits in a food processor until they reach a fine, sandy consistency. Set aside.
Melt Ganache: In a saucepan over low heat, combine the sweet cream, softened butter, and 150g of white chocolate. Stir constantly until fully melted and glossy.
Mix Filling: Remove from heat and stir in the biscuit crumbs, pistachio paste, and salt until a thick, uniform paste forms.
Initial Chill: Place the mixture in the freezer for 30 minutes until firm enough to handle.
Shape: Roll the chilled mixture into small, smooth balls (about 1 inch in diameter). Place on a parchment-lined tray and freeze for at least 1 hour. This deep chill is essential for a smooth coating.
Coat: Melt the remaining 300g of white chocolate in the microwave using 30-second intervals at 50% power, stirring until smooth. Dip each frozen ball into the chocolate, tap off the excess, and return to the tray.
Decorate: While the coating is still wet, sprinkle with crushed pistachios.
Set: Refrigerate or freeze until the outer shell is completely hard and set.