The Wonderffle Stuffed Waffle Iron cooks best with a slightly thick batter. You want something with the viscosity of, say, molasses (in the wintertime). In other words, you want your batter to flow but not as rapidly as those out-of-the-box waffle mixes. Granted, if you do choose to use a Bisquick waffle mix, for example, you might want to consider slightly reducing the amount of liquid the recipe calls for.
Now I don't normally take the effort to do anything fancy with my batters such as separate egg yolks from whites and whatnot. If you're determined to achieve the texture such techniques claim to achieve, Ree Drummond has what you're looking for.
As for me, my technique is to simply add the dry ingredients to a bowl one at a time, add the wet ingredients, and mix. Done.
So, first assemble everything you'll need to make your batter. This includes mixing bowl, spatula, and your wet and dry ingredients.
Now pour your flour, baking powder, sugar, and salt into a large mixing bowl...
...and mix them with your spatula.
Before you add your liquid ingredients, here's a word of advice: If you like to you buttermilk in your waffles, you will want to add 1/2 teaspoon of baking soda to your dry ingredients in order to balance the acidity of the buttermilk.
If you are like me and don't normally keep buttermilk in the fridge, you can just mix in 1 tablespoon of vinegar or lemon juice per 1 cup of milk and let it sit for 5 to 10 minutes until the milk curdles.
Yeah, I know this is a rather decadent batter, right?
Now, mix it all together with your spatula. You want to mix well, but not overzealously. You want it to keep some lumpiness.
Voilà, you're done!
Classic Waffle Batter
Yields about 4 stuffed waffles