Preheat the oven to 375°F. Coat the muffin tin with cooking spray or butter (optional: use muffin paper liner).
Smash the ripe bananas using a fork or potato masher. Add brown sugar, eggs, vanilla extract and mix. Add melted butter and mix.
In a bowl: add flour, salt, baking powder & baking soda while mixing everything together. (I highly recommend sifting the dry ingredients before adding to the wet ingredients, this will make the batter fluffy and not lumpy).
Combine wet and dry ingredients together. Mix until well combined. Add 1 cup of blueberries (if using frozen blueberries: do not let thaw, mix in while still frozen) then gently fold the batter with spatula until evenly distributed throughout the batter (do not over mix or use a mixer on this step, be gentle).
Use an ice cream scooper or spoon to measure even portions. As you carefully place each scoop of batter in the muffin tin (should be exactly 12 scoops). Sprinkle ½ cup of blueberries on top of the batter.
Bake for 25-28 minutes or until lightly golden brown. Let the muffins cool in the tin for about 5 minutes and then transfer them to a baking rack to finish cooling completely.