Preheat the oven to 175°C/350°F. Butter an 8x4-inch loaf pan (I used 8x3.5-inch) and line the bottom with parchment paper. Set aside.
Whisk flour, baking soda, baking powder and salt in a large mixing bowl.
In a 2-cup measuring cup, whisk together melted butter, buttermilk and eggs. Whisk in the brown sugar, followed by the mashed bananas.
Pour the wet ingredients into the dry ingredients. Fold with a large wooden spoon or spatula until just combined (until no flecks of flour is visible).
Scrape 1/2 of the batter into the prepared loaf tin and spread it evenly with a spatula or the back of a spoon. Drizzle 1/2 of the peanut butter onto the batter and swirl with a knife or skewer. Scrape the remaining batter into the tin spread it evenly. Drizzle with the remaining peanut butter and swirl it again.
Bake for 45 to 60 minutes until a skewer inserted into the center of the loaf comes out clean. (Check the loaf frequently after 30 minutes, if the top is getting overly browned, tent it with a piece of aluminium foil.) Let the loaf cool completely in the tin on a wire rack.
Turn the loaf out onto a serving plate. Slice and serve!
NOTES
Use overripe bananas - they are easier to mash, sweeter and more aromatic.
If you don't have buttermilk, you can: 1) Combine 75 grams milk and 5 grams vinegar. Let sit for 10 minutes and stir before using. 2) Use 40 grams plain yogurt and 40 grams milk.
Natural peanut butter contains at least 90% peanuts with no artificial sweeteners, colours or preservatives. You can use creamy peanut butter if you don't have natural peanut butter. Microwave the creamy peanut butter at 30-second intervals so that the peanut butter becomes runny, making it easier to drizzle.