Preheat oven to 350F. Line and grease a 12-cavity muffin pan, plus a 6-cavity pan if you have one.TIP - If you are working in batches, you can place the mixture in the fridge but once the first batch of muffins is cooling, pull the dough out to bring them back to room temperature.
Whisk together in a large bowl: all-purpose flour, baking powder, sea salt, and ground cinnamon. Set aside.
Add mashed bananas, buttermilk, brown sugar, sour cream, and vanilla to the bowl of a stand mixer and blend.
Add in eggs and mix until combined (about 30 seconds).
Slowly add in dry ingredients to incorporate.
Fold in 1 ½ cups of semi-sweet chocolate chips until they are distributed throughout.
Divide evenly into 18 muffins and bake for 15-18 minutes, and then cool on a wire rack for about 10-15 minutes before taking the muffins out of the tin.
Notes
When measuring your flour, use the scoop and sweep method to avoid dense muffins. Begin by aerating your flour with a spoon. Then, scoop the flour into the measuring cup using the same spoon. Once the measuring cup is full, sweep the excess off the top using the flat end of a butter knife. Do not pack your flour down or tap the cup on the counter. You never want to pack flour or scoop it straight from the bag, this is what makes your baked goods dense and crumbly.
You will definitely want to use overripe bananas for this recipe. As the banana becomes overripe, the starches are converted to sugar causing them to be sweeter and more flavorful. Your bananas can be black, brown, or yellow with brown spots – so long as the banana is nice and ripe your muffin will come out tasty. When the banana peel has developed freckles, they are ready. The riper the banana, the stronger the banana flavor in your muffin. Dark/black bananas are perfectly okay to use.
Store the muffins in an air-tight container or large zip top bag kept at room temperature for 4-5 days.
To freeze these muffins, store them in a large zip-top bag labeled with the date or individually wrapped with freezer wrap. Freeze for up to 3 months. Thaw at room temperature.