Heat oven to 350℉/177℃.
If using silicone molds, brush lightly with vegetable oil and place on sheet pan. Set aside.
If using metal molds, spray with cooking spray. Set aside.
In a medium bowl, whisk together all purpose flour, whole wheat flour, baking powder, baking soda, and salt. Set aside.
Finely chop walnuts. Set aside.
Peel and mash bananas. Set aside.
With an electric mixer, beat sugar and coconut oil until light and fluffy, about 2-3 minutes.
Add flour mixture alternately with eggs, beginning and ending with flour mixture.
Stir in bananas and walnuts.
Spoon batter into a piping bag and snip off end.
Pipe batter into donut molds, filling each ⅔ full.
Bake for 17-18 minutes, or until done.