Preheat the oven to 350 degrees F. Grease a 12-cup standard-size muffin tin.
In a large bowl, whisk the flour, sugar, baking soda, salt and walnuts together. In a separate bowl, combine the bananas, yogurt, eggs, butter and vanilla. Fold the wet ingredients into the dry until just combined. Scoop the batter evenly between the muffin tins.
Bake until the muffins are golden brown on top, approximately 20-25 minutes. Serve warm or at room temperature.
Store leftovers in an airtight container or sealed plastic bag at room temperature for up to three days. You can also freeze them in a freezer bag for up to three months.