Preheat the oven to 350°F and grease your preferred baking pan or mold. I use a silicone mini donut mold, but you can also use an 8x4 inch loaf pan or a 12-cup muffin tin.
In a small saucepan over medium heat, melt the butter and cook, stirring occasionally, until it turns golden brown and smells nutty, about 10 minutes. Be careful not to burn it. Let it cool slightly.
In a mixing bowl beat sugar and browned butter adding in one egg at a time, followed by the bananas.
In a small bowl, mix the flour, baking soda, salt and add the nuts if desired.
Add dry ingredients, ½ cups at a time, to the banana mixture. Once all incorporated add in sour cream and vanilla.
Pour the batter into the prepared pan or mold.
To make the streusel, combine the brown sugar, flour, cinnamon, and salt in a small bowl. Use a fork or your fingers to work the butter into the mixture until clumps form. Sprinkle evenly over the bread batter.
Bake for 34 to 45 minutes for mini donuts, or 1 hour for a loaf, or until a toothpick inserted into the center comes out clean.
Let the banana bread cool slightly in the pan or mold before transferring to a wire rack to cool completely.
Enjoy as is, or decorate with your favorite glaze, chocolate, or sprinkles.