Preheat your Belgian waffle iron according to manufacturer's instructions, allowing at least 5 minutes for it to reach full temperature.
In a large mixing bowl, whisk together the flour, cornstarch, sugar, baking powder, baking soda, and salt until evenly combined with no lumps.
In a separate bowl, whisk together the buttermilk, eggs, and vanilla extract until smooth and well blended.
Pour the wet ingredients into the dry ingredients and stir gently with a wooden spoon or spatula until just combined. Some small lumps are fine and expected.
Add the melted butter and fold it into the batter with 4-5 gentle strokes. Don't overmix — a few streaks of butter are okay. Let the batter rest for 5 minutes.
Lightly spray or brush the preheated waffle iron with cooking spray or oil. Pour the recommended amount of batter for your iron size (usually 1/2 to 3/4 cup for Belgian irons) onto the center.
Close the lid and cook without opening until the waffle is deep golden brown and crispy, typically 4-6 minutes depending on your iron. Wait until steam stops escaping before checking.
Carefully remove the waffle using a fork or silicone tongs and place on a wire rack or directly on an oven rack set to 200°F to keep warm and crispy while you cook the remaining batter.
Repeat with remaining batter, lightly spraying the iron between waffles if needed. Serve immediately with your favorite toppings.
