In a large bowl, whisk together the flour, sugar, baking powder, and salt.
In a separate medium bowl, whisk together the peanut butter and coconut oil.
Whisk the egg and milk into the peanut butter mixture until combined.
Pour the wet ingredients into the dry ingredients and stir just until no dry streaks remain.
Fold the mashed banana into the batter.
Preheat a waffle maker to medium heat and lightly coat it with nonstick cooking spray.
Pour about ¾ cup of batter onto the center of the waffle maker and close the lid. (Note: You may need to adjust the amount of batter you add depending on the size of your waffle maker.)
Cook for about 4 to 5 minutes, until the steam coming from the sides slows to a wisp and the waffle is golden brown. Repeat the process to cook all of the waffles (the batter should yield 4 large waffles in total).
As you cook the waffles, transfer finished ones to a wire rack to cool slightly.