Preheat oven to 350°F (175°C). Line a muffin tin with 6 liners or grease well.
Brown butter over medium heat, then let cool for 5–10 minutes.
In a small bowl, whisk together flour, baking soda, salt, and cinnamon.
In a large bowl, whisk browned butter with sugars until smooth. Add egg and vanilla, then whisk again.
Stir in mashed bananas.
Gently fold dry ingredients into wet ingredients until just combined (don’t overmix). If using nuts, toss them with a tiny bit of flour before folding in.
Divide batter evenly among the 6 muffin cups.
Make streusel: combine flour, brown sugar, butter, cinnamon, and salt with a fork until crumbly. Sprinkle evenly over muffins.
Bake for 30-35 minutes, or until a toothpick comes out with just a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a wire rack.
