Preheat the oven to 375. Coat a 12-cup muffin tin with cooking spray.
Measure 1/2 cup of the walnuts into a food processor and grind to a fine meal. Place the ground walnuts, flour, chocolate chips, baking powder, cinnamon, and salt in a large bowl and stir until thoroughly combined.
Combine the brown sugar, oil, yogurt, milk, egg, banana, and vanilla extract in a medium bowl and stir until smooth. Add the banana mixture to the flour mixture and stir until thoroughly combined. Stir in the remaining 1 cup walnuts and mix well (the batter will be thick).
Fill the muffin cups three-fourths full and bake for 13 to 15 minutes, or until the tops spring back lightly when touched. Remove the muffins from the pan and let cool on a rack.