Preheat your oven to 350°F and line a 9x5-inch loaf pan with parchment paper.In a medium bowl, whisk together flour, baking powder, baking soda, ground cinnamon, salt and oats until evenly blended.

Melt chocolate in a small bowl in the microwave at 50% power in short bursts with frequent stirring. Set aside.In a large bowl, mash the banana with a fork until smooth. Stir in sugar, then whisk in sour cream, eggs, vegetable oil, peanut butter and vanilla. Add dry ingredients all at once and stir together gently with a wooden spoon until just combined. Do not over-mix. A few small lumps are just fine.

Add about 1 cup of batter to the bowl with the still-warm melted chocolate and stir to blend evenly. Dollop peanut butter batter alternately with chocolate batter into prepared pan and swirl together with a knife.

Bake for 45-55 minutes until golden brown and slightly firm to the touch. Transfer pan to a wire rack and let cool for 10 minutes before transferring to the rack to finish cooling.