Preheat oven to 350°F (175°C). Grease a 9x5 inch (23x13 cm) loaf pan or line with parchment paper.
Brown the butter: Melt the 115g of butter in a small saucepan over medium heat. Stir until it foams, turns golden, and smells nutty (about 5–7 minutes). Set aside to cool slightly.
In a large bowl, whisk together mashed bananas, brown sugar, eggs, vanilla, and the browned butter.
In another bowl, whisk together flour, baking soda, salt, and cinnamon.
Stir the dry ingredients into the wet just until combined. Fold in optional walnuts or chocolate chips.
Make the crumble: Use a fork to mix butter, sugar, flour, and cinnamon until crumbly.
Pour batter into the pan, then sprinkle the crumble on top.
Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean.
Let cool 10–15 minutes in the pan before transferring to a wire rack.
Notes
You can just use melted butter or a light flavored oil in place of the brown butter. The crumble will lose its crispiness overnight, but is still delicious.