In a bowl, whisk together flour, cornstarch, sugar, and salt. Gradually mix in vegetable oil and enough water to create a smooth batter. Let the mixture rest for 30 minutes. Heat a nonstick pan over medium and pipe thin layers of the batter onto it. Cook until dry but not browned, then transfer to a towel. Repeat until all the batter is used, then allow the kataifi to cool.

Melt white chocolate with double cream in the microwave for about 30 seconds, stirring until smooth. Blend ground pistachios and a pinch of salt into a fine paste, then fold into the melted chocolate. Toast the cooled kataifi with butter over medium heat until golden, about 5 minutes, and mix it into the pistachio cream. Chill for 1 hour.

Scoop and roll the pistachio cream mixture into small balls, about 1 inch in diameter. Melt milk chocolate in a heatproof bowl over simmering water until fully liquid. Dip each ball into the melted chocolate, allowing excess to drip off, then place on parchment paper. Sprinkle tops with chopped pistachios before refrigerating until chocolate is set.