Preheat your oven to 350°F (175°C). Grease a muffin tin or line with muffin liners.
In a large bowl, whisk together the whole wheat flour, baking soda, and salt.
In another bowl, mix the mashed bananas, egg, honey, Greek yogurt, melted coconut oil, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Fold in the chopped walnuts.
Divide the batter evenly among the muffin cups.
Bake for 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Allow the muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.