Preheat the oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners. Mash the bananas in a large bowl, then whisk in melted butter, brown sugar, egg, vanilla, and yogurt until smooth.

In a separate bowl, whisk together flour, oats, baking soda, and salt.

Fold the dry ingredients into the wet mixture until just combined. If using, fold in walnuts or chocolate chips.

Divide batter evenly among the muffin cups, filling each three-quarters full. Bake for 18-22 minutes, until golden and a toothpick inserted in the center comes out with moist crumbs.

Let cool in the tin for 5 minutes, then transfer to a wire rack. Serve warm or at room temperature.