Instructions:
- Preheat oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line it with parchment paper, leaving an overhang on the sides.
- In a medium bowl, whisk together the flour, granulated sugar, brown sugar, baking soda, salt, and cinnamon until thoroughly combined. Set aside.
- In a separate large bowl, mash the ripe bananas until mostly smooth with a few small lumps remaining. Add the softened butter and cream together lightly.
- Beat in the eggs one at a time until incorporated, then stir in the vanilla extract.
- Gradually add the dry ingredient mixture into the wet mixture. Mix just until no dry streaks of flour remain. Do not overmix.
- In a small bowl, combine the peanut butter, powdered sugar, and 1 teaspoon of milk. Mix until smooth and thick. Add the final teaspoon of milk only if the mixture is too stiff to drizzle.
- Pour half of the banana batter into the prepared loaf pan. Dollop the peanut butter mixture evenly over the batter, followed by the remaining banana batter.
- Use a butter knife or skewer to gently cut through the batter in an 'S' shape 3-4 times to create a swirl effect. Do not over-swirl.
- Bake for 55–65 minutes, or until a wooden skewer inserted into the centre comes out clean or with moist, tender crumbs attached.
- Let the loaf cool in the pan for 15 minutes before carefully lifting it out onto a wire rack to cool completely before slicing.