Preheat oven to 350°F. Butter a 9-by-5-inch loaf pan. Cut a 9-inch-wide strip of parchment and press it into the pan to cover the bottom and long sides, leaving at least a 1-inch overhang on the long edges; if the short ends are unlined, butter them.

In a large bowl, sift or whisk together 1 3/4 cups all-purpose flour, 1 teaspoon baking powder, 1/2 teaspoon baking soda, and 1/4 teaspoon salt. Set aside.

In a medium bowl, mash 2 large ripe bananas until mostly smooth. Whisk in 2 large eggs until combined.

Add 1/2 cup granulated sugar, 1/2 cup packed light brown sugar, and 1 teaspoon vanilla extract to the banana–egg mixture; whisk to combine.

Whisk 1/2 cup (1 stick) unsalted butter, melted and slightly cooled, into the wet mixture until incorporated.

Pour the wet ingredients into the bowl with the dry ingredients and whisk gently until just combined and no streaks of dry flour remain. Do not overmix.

Transfer about half of the batter to a second bowl. To one bowl of batter, add 2 tablespoons black sesame tahini and 2 tablespoons black sesame powder or finely ground black sesame seeds; stir until evenly incorporated.

Spoon alternating scoops of plain batter and black sesame batter into the prepared pan to build up layers.

Using a bamboo skewer or butter knife, swirl the batters 3 or 4 times to create a marbled pattern; avoid over-swelling the swirl.

Sprinkle the top with additional granulated sugar (to your preference) and bake for about 50 minutes, or until a skewer inserted near the center comes out mostly clean with a few moist crumbs attached.

Remove the pan from the oven and place it on a wire rack to cool. When the loaf is cool enough to handle (about 10–15 minutes), run a thin metal spatula along the short unlined sides if needed, then use the parchment overhang to lift the loaf from the pan and transfer it to a cutting board. Cool completely before slicing.

Store the cooled banana bread in an airtight container at room temperature for up to 3 days.