Preheat the oven:
Preheat your oven to 350°F (175°C). Line a muffin tin with paper liners or grease it with butter or non-stick spray.

Mix dry ingredients:
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.

Prepare wet ingredients:
In a large bowl, cream together the softened butter and sugar until light and fluffy (about 2-3 minutes).
Add the eggs one at a time, beating well after each addition.
Stir in the mashed bananas and vanilla extract.

Combine wet and dry ingredients:
Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk (or regular milk), beginning and ending with the dry ingredients. Mix until just combined—be careful not to overmix.
If desired, fold in chopped walnuts or chocolate chips.

Fill muffin tin:
Divide the batter evenly among the muffin cups, filling each about 2/3 full.

Bake:
Bake in the preheated oven for 18-22 minutes, or until a toothpick inserted into the center comes out clean or with just a few crumbs.

Cool and enjoy:
Allow the muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.