Preheat the oven to 350°F (175°C). Grease the loaf pan or line it with parchment paper for easy removal.
In a mixing bowl, mash the ripe bananas with a fork or whisk until smooth. You should have about 1 ½ cups of mashed bananas.
Add the peanut butter, granulated sugar, brown sugar, eggs, and vanilla extract to the mashed bananas. Mix well until all ingredients are combined and creamy.
In another bowl, combine the all-purpose flour, baking soda, salt, and cinnamon (if using). Stir the dry ingredients together until well mixed.
Gradually add the dry ingredients to the wet mixture, folding gently until just combined. Avoid overmixing; it's okay if there are a few lumps.
If desired, fold in chopped peanuts or chocolate chips for added texture and flavor.
Pour the batter into the prepared loaf pan, spreading it evenly.
Bake in the preheated oven for 55-60 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the loaf from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely.