Preheat the oven to 425° F. Line the muffin pan with paper liners and lightly grease with non-stick spray. Set aside.
For buttermilk, add white vinegar to room-temperature milk, mix well, and set aside. If you have store-bought buttermilk, use it instead of milk and vinegar mixture.
Add flour, baking powder, baking soda, salt, and ground cinnamon in a medium bowl and whisk until well combined.
In a large mixing bowl, whisk the mashed bananas, melted butter, brown sugar, and vanilla until well combined.
Now, add the flour mixture and buttermilk and mix until just combined. Do not overmix.
Fold in chocolate chips into the batter and mix gently to distribute evenly.
Spoon the batter into the muffin pan, filling each tin all the way to the top.
Bake for 5 minutes at 425°F (220°C), then, keeping the muffins in the oven, reduce the oven temperature to 350°F (180°C). Continue baking for 12-14 minutes or until a toothpick inserted in the center comes out clean.
Remove the muffins from the oven and let them cool in the pan for 5 minutes. Then, transfer them to a cooling rack and let them cool completely before serving.