Place 1/2 cup of butter in a small saucepan over medium heat. Melt the butter, then continue cooking while stirring constantly. The butter will foam up and then begin to show little brown specks. Once it smells nutty and looks golden brown, immediately remove it from the heat and pour it into a heat-proof bowl to cool for 10 minutes. Do not leave the sediment behind; those brown bits hold all the flavor.

In a small mixing bowl, combine the chopped pecans, flour, brown sugar, and cinnamon. Add the cold cubed butter and the maple syrup. Use a fork or your fingers to work the butter into the dry ingredients until the mixture looks like coarse crumbs. Set this aside in the refrigerator to keep it cold while you prepare the batter.

Preheat your oven to 350°F (175°C) and grease a 9×5 inch loaf pan. In a large bowl, whisk together the cooled brown butter and brown sugar until smooth. Add the egg and vanilla extract, beating until well combined. Stir in the mashed bananas until the mixture is uniform.

Sift the flour, baking soda, salt, and cinnamon directly over the wet ingredients. Use a spatula to gently fold the dry ingredients into the banana mixture. Stop mixing as soon as no more streaks of flour remain. Overmixing will lead to a tough, rubbery loaf, so keep your touch light.

Pour the batter into the prepared loaf pan and smooth the top with a spatula. Sprinkle the chilled maple pecan streusel evenly over the entire surface. Bake for 55 to 65 minutes, or until a toothpick inserted into the center comes out clean or with just a few moist crumbs. If the streusel begins to brown too quickly, tent the pan loosely with aluminum foil halfway through baking.

Remove the pan from the oven and let it cool on a wire rack for at least 15 to 20 minutes. This resting period allows the structure of the bread to set. Carefully lift the bread out of the pan and let it cool completely before slicing. Cutting into it while hot can cause the bread to crumble.