I start by adding the flour, baking powder, baking soda, salt, cinnamon, and nutmeg into a large bowl. A quick whisk here spreads the leavening evenly through the flour. This little step keeps you from biting into a pocket of baking soda later, which nobody wants.
In a separate bowl, I whisk the eggs until they're a bit frothy, then stir in the apple cider, melted butter, brown sugar, and vanilla. Whisking the eggs first helps the batter come together smoother. Make sure your butter isn't piping hot or it can scramble the eggs slightly.
I pour the wet ingredients into the dry ingredients and stir just until no dry streaks remain. Overmixing is the enemy here, since it makes waffles tough instead of light. A few small lumps in the batter are completely fine and even expected.
While I preheat the waffle iron, I let the batter sit on the counter for about five minutes. This little rest gives the baking soda time to react with the cider's acidity. You'll actually see some tiny bubbles forming on the surface.
I always preheat my waffle iron fully before adding any batter, since a cold iron leads to pale, soggy waffles. A light spray of nonstick cooking spray or a quick brush of melted butter keeps everything from sticking. Do this every round, not just the first one.
I ladle the batter into the center of the hot iron, close the lid, and let it cook until golden and crisp. Most waffle irons take three to five minutes depending on size and heat setting. Resist the urge to peek too early, since lifting the lid can tear the waffle in half.
Since waffle irons only make one or two at a time, I keep finished waffles on a wire rack in a 200°F oven while I finish the batch. Stacking them directly on a plate traps steam and makes them soggy. This little trick keeps every waffle just as crisp as the first one.