Heat oven to 350 degrees F.
Butter a 6-cup (9×5-inch) loaf pan or coat it with a nonstick cooking spray and set aside.
Melt butter in a large bowl and whisk in brown sugar until smooth, then stir in mashed banana.
Whisk in eggs and vanilla.
Sprinkle the surface of the batter evenly with salt, cinnamon, nutmeg (if using), baking soda, and baking powder, and whisk until fully dispersed.
Add flour and stir until combined.
Scrape batter into prepared loaf pan.
Sprinkle the top of the batter with the raw sugar.
Bake banana bread for 55 to 65 minutes.
Let cool in pan.
To store, leave the banana bread in the pan, uncovered.
