Add a little canola oil to your waffle maker, and spread it around with a paper towel to coat.
Preheat your waffle maker.
In a large mixing bowl, stir together the flour, baking soda, baking powder, salt, and sugar until fully mixed.
In a second bowl, beat together buttermilk, eggs, and melted butter. Fold in the cold soda water and any mix-ins like cocoa powder or mini chocolate chips.
Pour the buttermilk mixture into the flour mixture. Stir with a wooden spoon until it's just blended together.
Ladle the batter into your waffle maker. Follow your waffle maker instructions for timing.
Gently remove from the waffle maker. Keep warm on a baking rack at 190 F while you cook waffles with the remaining batter. If the waffles are softer than desired, place in your toaster to crisp.