Whisk the flour, cornstarch, sugar, baking powder, baking soda and salt in a medium bowl. In another bowl or large measuring jug, whisk the milk, vegetable oil, egg and vanilla together until blended. Pour the milk mixture into the flour mixture and whisk until just mixed. Set batter aside for 10 to 15 minutes so the baking powder has time to activate. Heat the oven to 200° F and heat the waffle iron. Pour the batter onto the heated waffle iron (ours takes about 1/2 cup) and cook until the waffle is crisp and golden brown. Immediately set the cooked waffle directly on the oven rack to keep it warm and crisp. Repeat with the remaining batter, holding the waffles in the oven (try not to stack them). Serve with butter, syrup and berries.