In a small mixing bowl, whisk together the flour, baking powder, baking soda, salt and ground cinnamon. Tip - You can not use this mixing bowl by sifting the dry ingredients through a fine mesh strainer set over the top of the wet ingredient bowl.
In another small mixing bowl, mash up the banana with a fork until it is smooth, creamy, and free of lumps. Add the egg yolk, sugar, sour cream, oil, and vanilla extract and mix together with a whisk until thoroughly incorporated and smooth.
Sprinkle in half of the dry ingredients over the top of the wet ingredients. Mix until just combined. Add the remaining dry ingredients and mix until just combined and no dry streaks remain. Then fold in the strained apples until just combined.
Scoop equal portions of batter into the prepared brownie pan wells. Tip - Use a cookie scoop for equal portions and less mess. Bake for 16 - 18 minutes, or until a toothpick inserted in the middle comes out mostly clean.
Cool the banana bread in the pan set on wire cooling rack for 5 minutes, then transfer the banana bread to cool on the wire rack completely.