In a large mixing bowl combine: the flour, sugar, baking powder, baking soda, and salt. Mix well.
Melt the butter in the microwave completely. Let it cool for a few minutes.
Measure out 1 cup of milk into a measuring cup with space to add 1 cracked egg. Mix well to combine. Once the butter has cooled a bit, add it as well. Stir to combine quickly before the butter hardens.
Make a well in the center of the dried ingredients and pour in the wet ingredients.
Mix it all together until it is just combined and there is no dry flour. Don't worry about it being lumpy, the lumps will cook out. You don't want to overmix the batter. Fold in the cubed apple.
Allow the batter to sit out while the waffle maker heats up. The batter will thicken some as the flour hydrates. I would let the batter sit for about 5 minutes..
Once the waffle maker is hot. Brush butter on both top and bottom of the waffle maker then add your batter. Cook until waffle can be easily removed without sticking and it appears completely set. Repeat this until you have cooked all the waffles.
Notes
Feel free to leave the skins on the apples, there is no need to peel if you don't want to. I found that leaving the skins on I didn't really notice.