Preheat oven to 350°F. Grease (1) 9x5x2.5-inch loaf pan and line with parchment paper.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with an electric mixer, combine mashed bananas, peanut butter, sugar, oil, eggs and vanilla and mix until thoroughly combined.
In a large bowl, whisk together the flour, baking powder, baking soda, and salt until combined.
Stir the dry ingredients into the banana mixture until just blended. Pour batter into the prepared pan.
Drizzle the remaining 2 tablespoons of peanut butter over the top of the batter. (If your peanut butter is too thick to drizzle, microwave it for a few seconds to loosen it up.) Use a skewer or chopstick to swirl the peanut butter into the top of the bread.
Bake in preheated oven for 70-75 minutes or until a toothpick inserted in the center of the loaf comes out clean. Check bread around the 60-minute mark; if the top is deeply golden, lightly tent with foil to prevent the top from getting too dark.
Let cool in the pan for about 10 minutes before using the parchment paper to lift the bread out onto a cooling rack. Allow to cool completely before slicing.
Notes
I prefer using creamy peanut butter for this recipe, but you could use crunchy peanut butter if you want some added crunch in the banana bread. I recommend using creamy peanut butter for the swirl on the top of the loaf, though.If you run out of brown sugar, you can make a simple brown sugar substitute to use in this recipe instead. If your brown sugar hardened in the pantry, check out my tips for how to soften brown sugar and save yourself a trip to the grocery store. Store your peanut butter banana bread at room temperature, well wrapped in foil, for up to 3 days. You could also extend its shelf life by storing it in the refrigerator for up to 5 days or freezing for 1-2 months.