Begin by preheating your oven to 350°F (175°C). Prepare a muffin tin by either lining it with paper liners or lightly greasing it with cooking spray to prevent sticking.
In a large mixing bowl, combine the mashed bananas with the melted butter, blending them together until smooth and creamy.
Gradually add the brown sugar and granulated sugar to the banana-butter mixture, stirring vigorously until the sugars are completely incorporated.
Incorporate the eggs one at a time, ensuring you beat well after each addition. Follow this by mixing in the vanilla extract until the batter is nearly homogenous.
In a separate bowl, sift together the all-purpose flour, baking soda, baking powder, and sea salt. This helps to aerate the flour and ensure even distribution of the leavening agents.
Slowly add the dry mixture to the wet ingredients, gently folding with a spatula. Mix until just combined; be careful not to over-mix as this could result in dense muffins.
Carefully fold in the semi-sweet chocolate chips and chopped walnuts (if using), distributing them evenly throughout the batter for a delightful texture.
Using a spoon or an ice cream scooper, fill each muffin cup with batter, ensuring you fill them about 3/4 full for the best rise during baking.
Place the muffin tin in the preheated oven and bake for 18 to 20 minutes, or until a toothpick inserted into the center of a muffin comes out clean, indicating they are fully baked.
Once baked, let the muffins cool in the tin for about 5 minutes. Then, transfer them to a wire rack to cool completely, allowing the muffins to set and develop their flavors.
Notes
For an elegant touch, dust the cooled muffins lightly with powdered sugar before serving.