Make the bread: Preheat oven to 350F and grease an 11" loaf pan and set aside.
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 1/2 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 1/2 cup batter until smooth. Drop chocolate mixture by the tablespoon on top of batter in loaf pan. Use a knife to gently swirl the two batters.
Bake banana bread at 350F 50-55 minutes or until a toothpick inserted in middle comes out clean. Cool bread completely on a wire cooling rack. Once cooled, remove from pan. Slice and enjoy!