Preheat oven to 350 F and line an 8"x4" loaf pan with parchment paper. Spray with nonstick baking spray, then set aside.
In a large bowl, whisk together bread flour, baking soda, and salt.
In a separate bowl, whisk together butter and sugars until thoroughly combined, about 1-2 minutes. Add mashed bananas, eggs, greek yogurt, and vanilla bean paste, then whisk until smooth.
Gently fold dry ingredients into wet ingredients just until barely combined and no streaks of flour remain. Pour half of banana bread mixture into prepared loaf pan.
Place peanut butter in a microwave safe bowl (I use a 2 cup measuring cup) and heat for 30-45 seconds in a microwave until smooth and creamy. Pour have of melted peanut butter on top, then use a clean knife to swirl into the batter. Repeat with remaining banana bread batter and peanut butter, swirling again with a clean knife.
Bake in the center of a preheated oven for 50-55 minutes, or until a cake tester inserted into the cake comes out clean. Rest for 10 minutes before moving to a cooling rack. Allow to cool completely before slicing.