Prep the pan and oven. Line a 9x5-inch loaf pan with parchment, letting it overhang for easy lifting. Lightly grease. Preheat the oven to 375°F (190°C).

Make the peanut butter swirl. In a small bowl, whisk peanut butter, powdered sugar, 1 tablespoon milk, and a pinch of salt until smooth and pourable. Add the second tablespoon of milk only if it’s too thick to drizzle.

Mash the bananas. In a large bowl, mash bananas until mostly smooth with small lumps for texture. You should have about 1 1/3 cups.

Whisk the wet ingredients. To the bananas, add eggs, oil, brown sugar, granulated sugar, buttermilk, and vanilla. Whisk until combined and glossy, about 30 seconds.

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

Bring it together. Add the dry mix to the wet. Stir with a spatula just until the flour disappears. Do not overmix; a few streaks are fine.

Layer and swirl. Pour half the batter into the pan. Drizzle half the peanut butter mixture over it. Add remaining batter and then the rest of the peanut butter. Use a butter knife to gently swirl in a figure-eight pattern. Don’t overdo it—3–4 passes are enough.

Top it off (optional). Add a few banana slices down the center, sprinkle peanuts and turbinado sugar if you like.

Bake with a head start. Bake at 375°F for 15 minutes to set the dome. Without opening the oven, reduce heat to 340°F (171°C) and continue baking 35–45 minutes, until a tester inserted in the center comes out clean or with a few moist crumbs. Total time is about 50–60 minutes.

Cool properly. Let the loaf cool in the pan 15 minutes, then lift out and cool on a rack at least 1 hour before slicing. This sets the crumb and keeps it from squishing.