Combine Wet Ingredients: Crack the egg into a small bowl and whisk it well until fully beaten, about 30 seconds. Add the mayonnaise and water, stirring until completely blended.
Add Dry Ingredients: Sprinkle the almond flour and baking powder over the wet mixture, stirring with a fork until no lumps remain, about 1 minute. The batter should be smooth and pourable, similar to pancake batter.
Rest the Batter: Let the batter sit for 2 minutes; it will thicken slightly and become easier to work with in the waffle iron.
Preheat the Iron: Heat your waffle iron to medium-high heat and lightly grease both sides with butter or cooking spray, about 2 minutes of heating.
Pour the Batter: Pour half the batter (roughly 3 tablespoons) into the center of the waffle iron, spreading it slightly with a fork if needed to reach the edges evenly.
Cook Until Golden: Close the lid and cook for 4 to 5 minutes, until the chaffle is golden brown on both sides and crispy to the touch. Do not open the lid during cooking, as this releases steam.
Remove and Cool Slightly: Carefully lift the chaffle out with a fork and place it on a wire rack for 1 minute to set and become extra crispy. Repeat with the remaining batter to make a second chaffle.