Lightly grease the waffle iron and heat according to the iron's directions. (I usually preheat mine to medium-high.)
Combine the flour, whole wheat flour, baking powder, salt, brown sugar, and millet (if using) in a mixing bowl. Stir well to combine.
In a separate bowl, whisk together the buttermilk, eggs, and vanilla.
Tempter a small amount of buttermilk mixture into the melted butter and whisk with a fork to combine. Then pour the butter into the buttermilk mixture and whisk thoroughly.PRO TIP: Tempering a small amount of the cold buttermilk into the warm melted butter evens the temperature and creates an emulsification that prevents the butter from seizing into small, shard-like bits when added to the buttermilk mixture. That said, if the butter does seize, do not worry about it.
Make a well in the center of the dry ingredients. Pour the liquid into the center and stir to combine. Do not over-mix the batter.
Cook the waffles according to your waffle iron's directions. It will probably take about 7 minutes, give or take, for them to fully cook and become golden brown.
Disclaimer: Nutrition information is provided for courtesy purposes only, and is an estimate not verified by medical or nutrition experts. Read the full nutrition disclaimer.