Step 1: Preheat your oven to 300°F (150°C) and line a large baking sheet with parchment paper. Draw a 9-inch circle on the paper as a guide, then flip the paper over.
Step 2: In a perfectly clean, grease-free bowl, beat the egg whites and cream of tartar on medium speed until soft peaks form.
Step 3: Increase the speed to high and add the superfine sugar one tablespoon at a time, whisking continuously until the mixture is stiff, thick, and very glossy.
Step 4: Sift the cornstarch over the meringue, then add the vinegar and vanilla extract. Gently fold these in with a spatula until just combined.
Step 5: Pile the meringue onto the prepared parchment paper within the circle guide. Use a spatula to create a slight well in the center and decorative swirls on the sides.
Step 6: Place in the oven and immediately reduce the temperature to 250°F (120°C). Bake for 60 to 75 minutes, or until the outside is firm and dry to the touch.
Step 7: Turn off the oven and leave the cake inside with the door slightly ajar to cool completely for at least 2 hours. This prevents the cloud cake recipe from cracking excessively.
Step 8: Once cool, top with whipped cream and fresh berries just before serving.