Simmer the aquafaba in a small saucepan until it reduces from 1 cup to 2/3 cup. Transfer it to a bowl and refrigerate until completely cold.
Preheat the oven to 225 degrees F. Line a baking sheet with parchment paper and trace a 9-inch circle on the underside as a shaping guide.
Beat the chilled aquafaba and cream of tartar with an electric mixer until soft peaks form. Add the superfine sugar 1 tablespoon at a time, beating well after each addition.
Continue beating until the meringue is thick, glossy, and no longer feels gritty. Beat in the cornstarch, vanilla extract, and xanthan gum just until incorporated.
Spread the meringue inside the traced circle, building the edges slightly higher than the center. Bake for 2 hours, then turn off the oven and leave the pavlova inside for 1 additional hour without opening the door.
Remove the pavlova and let it cool completely. Beat the heavy cream, powdered sugar, and vanilla extract until medium peaks form.
Spoon the whipped cream into the center of the cooled pavlova and top it with strawberries, blueberries, and raspberries. Assemble immediately before serving to preserve the crisp shell.
