Preheat your oven to 350°F (175°C) and line a standard muffin tin with paper liners or grease the cups lightly.

Mash the overripe bananas in a mixing bowl until smooth and creamy, without large lumps.

Mix in the melted butter, sugar, beaten egg, and vanilla extract until combined.

In a separate bowl, whisk together the all-purpose flour, baking soda, and salt until well combined.

Gradually add the dry ingredients to the banana mixture, folding them together gently until just combined.

Fold in any optional add-ins like chopped walnuts or chocolate chips.

Divide the batter evenly among the lined muffin cups, filling each about two-thirds full.

Bake in the preheated oven for 20-25 minutes until the tops are golden brown and a toothpick inserted comes out clean.

Let the muffins cool in the pan for about 5 minutes, then transfer to a wire rack to cool completely.