Preheat the oven to 350 degrees F. Butter a 9 x 5"loaf pan and set aside.
In the bowl of your mixer, beat the butter and sugar until fluffy. Scrape down the sides and add eggs, one at a time. Mix to combine. Add the vanilla and bananas, scrape down the sides, and mix for 15 seconds longer.
In a bowl, whisk together the flour, baking powder, and salt. Add to the banana mixture until just mixed.
Add half of the batter to the greased pan. Dollop on the peanut butter (press between your fingers, come on, get messy!). Top with the remaining batter.
Bake for 55-60 minutes, until a toothpick comes out with only a few moist crumbs.
Let cool for 20 minutes on a wire rack before turning out to cool completely.