Preheat oven to 350 degrees F. Grease an 8" round cake pan or an 8x4-inch loaf pan.
Whisk together the flour and baking soda. Add the rest of the ingredients and, using a rubber spatula or wooden spoon, mix until thoroughly combined. Pour batter into pan and bake for about 30 minutes, or until a knife inserted in center comes out clean. Allow to cool, then spread additional peanut butter on top for "frosting", if desired.