Pulse the biscuits in a food processor until they become fine crumbs with a sandy texture. Set aside.
In a saucepan over low heat, combine the heavy cream, softened butter, and 150 g white chocolate. Stir continuously until melted and smooth.
Remove from heat and mix in the biscuit crumbs, pistachio paste, and salt until fully combined into a thick paste.
Transfer the mixture to the freezer for 30 minutes until firm enough to shape.
Roll the chilled mixture into small balls, about 1 inch in diameter. Arrange on a parchment-lined tray.
Freeze the truffles for at least 1 hour to firm completely.
Melt the remaining 300 g white chocolate in the microwave in 30-second intervals at 50% power, stirring between each interval until smooth.
Dip each frozen truffle into the melted white chocolate, allowing excess coating to drip off. Place back onto the tray.
Immediately sprinkle with crushed pistachios and decorate with edible gold leaf if desired.
Refrigerate or freeze until the chocolate shell is fully set.