Preheat oven to 350 degrees. Lightly spray or line six wells of a standard size muffin pan, set aside.
In the bowl of a stand mixer, beat together melted butter and sugar. Add sour cream (or buttermilk) and mashed bananas, mix on low for 30 seconds. Add egg and vanilla, mix until just combined.
In a small mixing bowl, combine flour, oats, baking soda, and salt. Whisk together then with mixing speed on low, gradually add dry ingredients. Mix until just combined. Remove bowl from stand and fold in chocolate chips. Using a large scoop, fill each well ¾ of the way full (or a little more if you have some extra).
Place in oven and bake for 18-22 minutes. The tops should be lightly golden brown and they should be nice and firm. Remove from oven and allow to cool in pans for 2-3 minutes before removing and transferring to a wire rack to cool completely.
Notes
-You will want to double this recipe for 12 standard size muffins.