Prepare the Pistachio Paste: In a food processor, blend 200g of the shelled pistachios with 50g of granulated sugar until a fine, slightly oily paste forms. It should have a vibrant green colour and a smooth, creamy texture. This might take a few minutes, scraping down the sides as needed.

Make the Custard: In a large mixing bowl, whisk together the eggs, remaining 50g granulated sugar, plain flour, and salt until smooth and pale yellow. Gradually whisk in the milk and vanilla extract until fully combined, creating a thin, milky mixture.

Combine Custard and Pistachio: Gently fold the pistachio paste into the custard mixture. Whisk well until the custard is a uniform pale green colour, ensuring no streaks of plain custard remain. The aroma will be distinctly nutty and sweet.

Prepare the Croissants: Preheat your oven to 180°C (160°C fan/Gas Mark 4). Slice the day-old croissants in half horizontally. Brush the cut sides of the croissants generously with the melted butter, ensuring they are well coated and glistening.

Assemble the Bake: Arrange the buttered croissant halves in a single layer in a 9x13 inch baking dish. Pour the pistachio custard evenly over the croissants, making sure every piece is thoroughly soaked. Allow it to sit for 10-15 minutes, letting the croissants absorb the creamy liquid, becoming noticeably softer to the touch.

Bake to Perfection: Sprinkle the remaining 50g of chopped pistachios over the top. Bake for 30-35 minutes, or until the custard is set and the top is golden brown and bubbling gently at the edges. A skewer inserted into the centre should come out mostly clean. The kitchen will be filled with a wonderful sweet, nutty scent.

Serve: Remove from the oven and let it cool slightly for 5-10 minutes. Dust with icing sugar if desired before serving. The top should have a slightly crisp texture, while the inside remains wonderfully soft and creamy.