Prepare the pistachio cream: Place the shelled pistachios and caster sugar in a food processor. Blitz for 2-3 minutes until the mixture forms a fine, slightly sticky paste. You should hear the sound change from a loud grinding to a smoother, softer whir. The paste should feel slightly warm from the friction and smell intensely nutty and sweet.

Make the custard base: In a large mixing bowl, whisk together the eggs, double cream, whole milk, vanilla bean paste, and sea salt flakes. Whisk vigorously until the mixture is completely smooth and pale yellow in colour. You should see tiny bubbles forming on the surface as you whisk, indicating the eggs are well incorporated.

Combine the pistachio cream with custard: Add the pistachio paste to the custard mixture and whisk thoroughly until fully incorporated. The mixture will turn a beautiful, pale green colour and should have a smooth, silky consistency. Taste a tiny bit — it should be nutty, sweet, and slightly salty from the sea salt.

Prepare the croissants: Using a sharp serrated knife, slice the croissants horizontally in half, as if you were making a sandwich. Arrange the bottom halves in a single layer in a 9x13 inch baking dish that has been lightly greased with butter. The croissants should fit snugly — you may need to slightly overlap them.

Assemble the bake: Pour half of the pistachio custard evenly over the croissant bottoms. You should see the custard pooling slightly around the edges. Place the croissant tops on, cut side down, and pour the remaining custard over the tops. Gently press down with a spatula to help the croissants absorb the liquid. If using white chocolate, scatter the pieces between the croissant layers now.

Rest and soak: Let the assembled bake rest at room temperature for 15 minutes. During this time, you'll notice the croissants beginning to soften and absorb the custard. The tops should look slightly moist but not completely saturated. This resting period is crucial for the texture.

Bake to perfection: Preheat your oven to 180°C (160°C fan) or 350°F. Place the dish on the middle rack and bake for 30-35 minutes. After 20 minutes, check the colour — the edges should be turning golden brown. If the top is browning too quickly, loosely cover with aluminium foil for the remaining time. The bake is ready when the centre is just set (it should jiggle slightly, like a cheesecake) and a knife inserted into the centre comes out clean. The kitchen should smell of toasted nuts, butter, and vanilla.

Finish and serve: Remove from the oven and let cool for 5-10 minutes. Dust generously with icing sugar through a fine sieve. The sugar should create a beautiful white veil over the golden-green surface. Serve warm, spooning portions that show the soft, custard-soaked layers.