Preheat oven to 250 degrees. In a large mixing bowl, combine flours, sugar, baking powder, baking soda, and salt. Whisk in buttermilk, egg, and butter until batter is well combined and free of lumps.
In a large nonstick skillet over medium heat, swirl oil to coat bottom of pan. Working in batches, add batter in 1/4-cup portions; cook until golden brown and tiny air bubbles form evenly on top, 2 to 5 minutes.
Top with banana slices, dividing evenly; flip pancakes, and continue cooking until slightly puffed, 2 to 5 minutes (reduce heat if browning too quickly). Transfer to a baking sheet; keep warm in oven while you cook remaining pancakes.
Serve pancakes with maple syrup and walnuts, if desired.
I added 1 tsp of vanilla and reduced the sugar to half. The caramelization of the bananas make these pancakes my favourite! I won't ever be mashing the bananas again. My 1 year old LOVES them. Delicious!
Placing the sliced bananas on the pancakes while they're in the griddle makes them deliciously caramelized. It also makes them stick to the spatula. (I tried mixing them into the batter and it was a soggy mess. It's definitely worth the trouble to add them in the pan.)