Step 1: Preheat your oven to 250°F (120°C). Line a large baking sheet with parchment paper. Draw a 9-inch circle on the parchment paper (you can trace a dinner plate) and flip the paper over so the drawing is on the underside.
Step 2: In a very clean, large glass or metal bowl, beat the egg whites with an electric mixer on medium speed until soft peaks form.
Step 3: Gradually add the superfine sugar, 1 tablespoon at a time, continuing to beat on high speed until the meringue is glossy, thick, and forms stiff peaks. This should take about 8-10 minutes. When you rub a small amount between your fingers, it should feel smooth with no sugar granules.
Step 4: Gently fold in the white vinegar, cornstarch, and 1/2 tsp ground sumac using a spatula until just combined.
Step 5: Spoon the meringue onto the prepared parchment paper, mounding it within the traced circle. Create a slight well or dip in the center for the toppings.
Step 6: Bake for 1 hour 30 minutes, or until the meringue is crisp on the outside and sounds hollow when tapped. Turn off the oven and leave the meringue inside with the door ajar for at least 1-2 hours, or preferably overnight, to cool completely. This prevents cracking.
Step 7: While the meringue cools, prepare the strawberries. In a medium bowl, combine the sliced strawberries with 1 tbsp granulated sugar and 1/2 tsp ground sumac. Toss gently and let sit for 10-15 minutes to macerate and release their juices.
Step 8: In a cold bowl, whip the heavy cream and vanilla extract until soft peaks form. Do not over-whip.
Step 9: Once the meringue is completely cool, carefully transfer it to your serving platter. Spoon the whipped cream into the center of the meringue.
Step 10: Arrange the sumac-infused strawberries over the whipped cream. Sprinkle generously with chopped pistachios and fresh basil leaves. Serve immediately and enjoy your beautiful Strawberry Sumac Pavlova with Pistachios & Basil!