Step 1: Preheat your oven to 130°C (275°F). Line a large baking tray (approx. 25×35 cm or 10×14 inches) with parchment paper. Draw a rectangle on the parchment paper as a guide, then flip it over so the pencil marks are on the underside.
Step 2: In an impeccably clean, dry bowl (preferably glass or metal), whisk the room-temperature egg whites with an electric mixer on medium speed until soft peaks form.
Step 3: Gradually add the caster sugar, one tablespoon at a time, while continuing to whisk on high speed. Make sure each spoonful is fully incorporated before adding the next.
Step 4: Continue whisking until the meringue is very thick, glossy, and holds stiff peaks. When you rub a small amount between your fingers, it should feel smooth, with no grittiness from undissolved sugar.
Step 5: Gently fold in the white wine vinegar and cornflour using a spatula, being careful not to deflate the meringue.
Step 6: Spread the meringue evenly within your drawn rectangle on the parchment paper. You can create gentle swirls or peaks with the back of a spoon.
Step 7: Bake for 1 hour to 1 hour 15 minutes, or until the meringue is crisp on the outside and sounds hollow when tapped. The inside should be slightly soft and marshmallowy.
Step 8: Turn off the oven and leave the meringue inside with the door ajar (you can prop it open with a wooden spoon) for at least 2-3 hours, or ideally overnight, to cool completely and dry out.
Step 9: While the meringue cools, prepare the pistachio cream. Bring a small pot of water to a boil. Add the shelled pistachios and blanch for 1-2 minutes. Drain and immediately transfer to a bowl of ice water.
Step 10: Drain the pistachios and rub them vigorously with a clean kitchen towel to remove the skins. This helps enhance their vibrant green color and ensures a smoother cream.
Step 11: In a food processor, blend the peeled pistachios until they form a fine meal. Add the icing sugar and a pinch of salt and blend again.
Step 12: With the food processor running, slowly drizzle in the pistachio oil (if using) and a tablespoon of milk until a smooth, thick paste forms. Add more milk if needed to reach a spreadable consistency.
Step 13: In a separate large bowl, whisk the cold heavy cream with the vanilla extract until soft peaks form.
Step 14: Gently fold the pistachio paste into the whipped cream until just combined. Be careful not to overmix.
Step 15: Once the meringue base is completely cool and dry, carefully peel it off the parchment paper and transfer it to your serving platter.
Step 16: Evenly spread the pistachio cream over the meringue base, leaving a slight border if desired.
Step 17: Arrange fresh berries, a sprinkle of chopped pistachios, and a dusting of icing sugar over the cream. Serve immediately.