Preheat your oven to 350℉. Grease your loaf pans well.
In a mixing bowl, combine the flour, corn starch, baking soda, and baking powder together.
In your mixer, cream together the butter, oil, both sugars, and peanut butter. It'll take 2-3 minutes on medium speed. Add int he eggs one at a time, mixing a bit in between them.
At this point, stop using the mixer. Add in the flour mixture and fold it with your rubber spatula until ti's just combined. Try to avoid over-mixing to help keep the cake light.
Divide the batter evenly into the loaf pans. Bake for about 30-35 minutes, or until a toothpick comes out clean.
Let the cakes cool in the loaf pans for at least 15 minutes before turning them out onto a cooling rack.
If you're making a ganache, just melt and combine the three ingredients until they're smooth and mixed. Let it cool a bit, then spread or pour it over the top of the cooled loaf cakes. Let it harden a bit, then slice and serve!