Preheat oven to 350 degrees F (177 degrees C). Position rack in center of oven. Line the muffin pan with paper liners or grease with butter or a vegetable spray. Set aside.
Place the two bananas in a food processor or blender and puree. You will end up with about 1 cup (240 ml) of banana puree. Note: If you do not have a food processor just use a fork or else your hands to mash the bananas until they are pureed.
In a small saucepan melt the butter. Set aside.
In a medium sized bowl whisk together the brown sugar, eggs, vanilla extract and banana puree. Add the melted butter to the brown sugar mixture and stir to combine.
In another large bowl combine the flour, baking powder, baking soda, cinnamon, and salt. Gently fold in the berries, making sure they are coated with flour. This helps to prevent the berries from sinking during baking. Do not overmix or the blueberries will end up coloring the batter blue. Add the wet ingredients to the dry ingredients and stir only until the ingredients are just combined. Do not overmix the batter or tough muffins will result.
Fill each muffin cup about two thirds full of batter, using two spoons or an ice cream scoop. Fill any unused muffin cups halfway with water to prevent warping of the pan and/or over browning of the muffins. Place in the oven and bake until a tester inserted in the center of a muffin comes out clean, 20 to 25 minutes. Transfer to a wire rack to cool.
Makes 12 regular sized muffins.
2 large ripe bananas, peeled
1/2 cup (1 stick) (113 grams) unsalted butter
1 cup (215 grams) light brown sugar
2 large eggs, lightly beaten
1 teaspoon pure vanilla extract
1 cup fresh or frozen blueberries or strawberries (cut into pieces)
2 1/4 cups all-purpose flour
1 1/4 teaspoons baking powder
1/4 teaspoon baking soda
1 teaspoon ground cinnamon
1/2 teaspoon salt