Preheat the oven to 350°F and grease a loaf pan.
In a large bowl, beat bananas, sugar, peanut butter, butter, milk, eggs, and vanilla until smooth. Fold in flour, baking powder, baking soda, and salt until batter is just combined.
Reserve ½ cup of batter and place in a medium bowl. Scrape remaining batter into prepared pan and set aside.
Whisk melted chocolate and espresso into reserved batter until smooth. Place chocolate mixture by the tablespoon on top of batter in a loaf pan. Use a knife or toothpick to gently swirl the two batters.
Bake banana bread for 50-55 minutes or until a toothpick inserted in middle comes out clean.
Let set and once cooled, remove from pan. Slice and enjoy!