Combine all wet ingredients in a large mixing bowl. Stir until everything is well incorporated.
In a separate bowl, whisk together dry ingredients except the add-ins (whole wheat flour, cornmeal, baking powder, cinnamon, and nutmeg).
Add dry ingredients to wet and fold the batter together to combine, being careful not to over-mix. Fold in add-ins (if using).
Let the batter rest for at least 10 minutes. Preheat the oven to 350°F/180°C. Line a loaf pan with parchment paper.
Once the batter has rested and the oven is preheated, pour the batter into the loaf pan.
Bake for 60-75 minutes, until a skewer inserted in the center of the loaf comes out clean. For an 8"x4" pan, it will take about 75 minutes. For a 9"x5" pan, it should be ready around 60 minutes. Tent the loaf with foil if it's browning too quickly (check around the 45 minute mark).
Place the loaf pan on a wire rack and let the bread cool in the pan for 10 minutes. Then carefully remove the bread from the pan (grab the sides of the parchment paper), place on the wire rack, and gently remove the parchment paper. Let the bread cool on the wire rack for at least 1 hour before slicing.
To store the bread, let it cool to room temperature, then place in an airtight container. The bread can be kept at room temperature if you're serving it that day, otherwise store in the fridge for up to a week. To freeze, cut the bread into slices, place in freezer-safe bags (don't stack them or they'll stick), and freeze for up to two months.
Notes
1) You will need about 2½ large bananas. You can use the remaining banana to top the loaf, freeze the banana, or just eat it. 2) You can swap the peanut butter for almond butter, walnut butter, or sunflower seed butter. However, be aware that sunflower seed butter can turn the loaf green due to a reaction with the chlorophyll. 3) You can't taste the miso, but it adds a very subtle salty, umami flavor to complement the sweetness of the loaf. Make sure you use plain miso (some have dashi added). If you don't have miso, you can add a pinch of salt instead.4) I used fine grind cornmeal which has a texture more similar to flour and produces a softer, cake-like texture. Medium grind cornmeal will likely produce a more crumbly texture.