Step 1: Prep the Oven and Tray
Preheat your oven to 300°F (150°C). Line a large flat baking sheet with parchment paper. Use a pencil to trace an 8-inch circle on the paper, then flip the paper over so the pencil mark does not transfer to your food.
Step 2: Whip the Egg Whites
Ensure your mixing bowl and whisk attachment are completely clean and grease-free. Whip the room-temperature egg whites on medium speed until soft peaks form.
Step 3: Add the Sugar
Increase the mixer speed to medium-high. Add the caster sugar one tablespoon at a time, waiting a few seconds between each addition. Continue whipping until the mixture is thick, glossy, and holds stiff peaks. (Test by rubbing a little mixture between your fingers; it should feel completely smooth, not gritty).
Step 4: Fold in the Stabilizers
Remove the bowl from the mixer. Sprinkle the cornstarch, white vinegar, and vanilla extract over the glossy meringue. Use a silicone spatula to gently fold the ingredients together. Do not overmix, or you will deflate the egg whites.
Step 5: Shape and Bake
Spoon the mixture onto your parchment paper, using your traced circle as a guide. Smooth the edges and create a slight dip in the center to hold your toppings later. Place the tray in the oven and immediately reduce the heat to 225°F (110°C). Bake for 90 minutes.
Step 6: Cool Completely
Turn the oven off, but do not open the door. Leave the baked meringue inside the oven to cool completely for at least a few hours, or ideally overnight. This prevents sudden temperature changes that cause cracking.