Heat the oven to 250 F and line a large baking sheet with parchment paper. Draw a 9-inch circle on the paper, then turn the paper over.
Beat the egg whites and salt until soft peaks form. Add the superfine sugar 1 tablespoon at a time, beating until the meringue is thick, glossy and no longer gritty.
Fold in the cornstarch, vinegar and vanilla. Spoon the meringue inside the marked circle, building taller sides and a shallow well in the center.
Bake for 1 hour 30 minutes, then turn off the oven and leave the pavlova inside with the door closed until completely cool, about 2 hours.
Beat the heavy cream, powdered sugar and vanilla to medium peaks. If the dulce de leche is very firm, stir in 1 tablespoon of the whipped cream to make it easier to spread.
Spread the dulce de leche inside the pavlova and add the whipped cream. Arrange the bananas on top, then finish with dark chocolate and flaky sea salt just before serving.