Preheat oven to 325 degrees. Lightly coat a doughnut pan with cooking spray and set aside. (Note: If you don’t have a doughnut pan, you can either make these in muffins or pour batter into a bundt pan for one giant doughnut.)
In a large bowl, whisk together flour, baking soda, and salt. Set aside.
In a medium bowl, combine coconut oil, bananas, agave syrup, yogurt, egg, and vanilla, and whisk until smooth.
Add wet mixture to dry mixture, and stir until just combined.
Pour batter into a resealable plastic bag (or a pastry bag). Cut off one corner of the bag, and squeeze batter into prepared doughnut pan, making sure each indentation is about 3/4 full. Top with walnuts pieces, and lightly press them into batter.
Bake for 14 to 16 minutes, or until doughnuts are golden and spring back when touched. Remove from oven and cool in pan for a few minutes, then carefully turn out onto a wire rack and cool fully. (Note: Baking time will vary if using a different type of pan.)