In the bowl of a stand mixer, whip the 3 egg whites until stiff peaks form. This will give extra fluffiness to your waffles. Once whipped, set the bowl aside for later use.

In a large mixing bowl, combine the all-purpose flour, dark brown sugar, baking powder, baking soda, ground cinnamon or pumpkin pie spice, and kosher salt. Whisk thoroughly to ensure the dry ingredients are evenly combined.

In a separate small bowl, mix together the milk, pure canned pumpkin, egg yolk, melted and cooled unsalted butter, and apple cider vinegar. Stir until the mixture is smooth and the ingredients are well incorporated.

Pour the pumpkin mixture (from Step 3) into the bowl with your dry ingredients (from Step 2). Stir gently until everything is just combined and there are no large pockets of flour. Be careful not to overmix—this keeps the waffles light and tender. I like to stop mixing as soon as I don't see any dry streaks left.

Carefully fold the whipped egg whites (from Step 1) into the waffle batter (from Step 4) using a spatula. Do this gently to avoid deflating the egg whites, which will help give your waffles a beautifully airy texture.

Preheat your waffle maker according to the manufacturer's instructions. Use the batter immediately, portioning it into the waffle maker and cooking until golden brown and cooked through. Serve the waffles warm. For an extra fall touch, I like to sprinkle a little extra cinnamon on top before serving.