Preheat oven to 350F. Line a 9×9" cake pan with parchment paper and set it aside.
In a mixer with the paddle attachment, add ripe bananas. Mix until bananas are mashed. Then add softened butter and espresso powder. Mix until well combined.
Add in granulated sugar, cocoa powder, vanilla extract, honey, baking powder, and baking soda. Mix together until well incorporated, about 2 minutes.
Add in the salt, eggs, and flour. Mix again until no clumps are left. Pour batter into the prepared cake pan.
In a microwave-safe bowl, add in the 1/2 cup peanut butter. Microwave for 20 seconds until smooth and liquidy. Then drop spoonfuls of the peanut butter on top of the batter. With a knife, swirl the peanut butter into the batter. Do not over-mix the peanut butter into the batter as you still want the swirls to be visible and stay on top.
Bake for 50-60 minutes or until a knife inserted into the center comes out clean with a few crumbs attached.