Step 1: Preheat your waffle iron to its highest setting or the ‘waffle’ setting. A properly heated iron is the first secret to crispy buttermilk waffles.
Step 2: In a large bowl, whisk together the flour, cornstarch, sugar, baking powder, baking soda, and salt until well combined.
Step 3: In a separate medium bowl, whisk the eggs until light. Add the buttermilk, melted butter, and vanilla extract, whisking until the mixture is uniform.
Step 4: Pour the wet ingredients into the dry ingredients. Using a spatula, gently fold the mixture together. Stop as soon as the flour is incorporated; do not overmix the buttermilk waffles batter.
Step 5: Let the batter rest for 5-10 minutes. This allows the flour to hydrate and the leavening agents to begin working, ensuring fluffier buttermilk waffles.
Step 6: Lightly grease the waffle iron if necessary. Pour the recommended amount of batter (usually about 1/2 to 3/4 cup depending on your iron size) onto the center and close the lid.
Step 7: Cook until the waffle iron stops steaming or the indicator light turns on. For extra crispy buttermilk waffles, leave them in for an additional 30-60 seconds.
Step 8: Remove carefully and serve immediately, or place on a wire rack in a low oven (200°F) to keep warm and crisp while you finish the rest.