Preheat your oven to 350ºF and grease an 8×8-inch square baking dish with nonstick cooking spray.
In a large bowl, combine all dry ingredients except for the berries and whisk them together. Then, add the wet ingredients into the dry mixture and stir well with a wooden spoon. Finally, gently fold in the berries until evenly distributed.
Transfer the batter into the prepared baking dish and sprinkle additional berries and oats on top, if desired.
Bake at 350ºF for approximately 40 minutes or until the center is fully cooked.
Allow the breakfast cake to cool for at least 15 minutes before slicing and serving.