In a medium bowl, whisk together the flour, baking soda, baking powder and salt. Set asdie.
In a large bowl with an electric mixer or the bowl of a stand mixer with a paddle attachment, beat the butter on medium speed until light and fluffy, about 2 minutes. Add the brown sugar and beat until fluffy, about 2 minutes more. Add the egg and vanilla. Mix just until combined.
Add the flour mixture to the batter in 3 parts, mixing just until combined after each addition. Add in the mashed banana and sour cream, mixing on low for about 30 seconds. Using a spatula, fold in the chocolate chunks until combined.
*If you have the time, cover the batter and refrigerate it for a few hours, for absolute best results. If not, go ahead and bake right away.
Preheat oven to 425F.
Line 6 muffin cups with parchment liners. Divide batter between the muffin liners. Top with a bit of rolled oats if you like.
Place in oven AND IMMEDIATELY REDUCE OVEN TO 325F. Bake for about 25 minutes, or until muffins are golden brown and a tester comes out clean.
Allow to cool in the pans for 5 minutes, then remove from pan, in wrappers, to a cooling rack to cool completely.