Prep the pan and oven: Heat the oven to 350°F (175°C). Line a 9x5-inch loaf pan with parchment, leaving a little overhang for easy lifting. Grease the pan and parchment.

Mix the swirl: In a small bowl, combine 1/3 cup brown sugar and 1 1/2 teaspoons cinnamon. Set aside.

Mash the bananas: In a large bowl, mash the bananas until mostly smooth. A few small lumps are fine.

Whisk in wet ingredients: Add granulated sugar, 1/4 cup brown sugar, oil (or melted butter), eggs, and vanilla. Whisk until blended and glossy.

Combine dry ingredients: In a separate bowl, whisk flour, baking soda, baking powder, salt, and 1 teaspoon cinnamon.

Bring the batter together: Add the dry ingredients to the wet. Stir gently until just combined and no dry streaks remain. Do not overmix. Fold in nuts or chocolate chips if using.

Layer the swirl: Pour half the batter into the pan. Sprinkle on all the cinnamon-sugar swirl mixture in an even layer. Top with the remaining batter and gently smooth the surface.

Optional top: For a lightly crackly top, sprinkle 1–2 teaspoons granulated sugar over the batter.

Bake: Bake for 55–65 minutes, or until a toothpick comes out mostly clean with a few moist crumbs. If the top is browning too fast, tent loosely with foil for the last 10–15 minutes.

Cool: Let the bread cool in the pan for 10–15 minutes, then lift it out and cool on a rack. For the neatest slices, wait until it’s just warm or room temperature before cutting.