Preheat oven to 350 degrees F Spray a 9 x5 inches loaf pan, then lined it with parchment paper. Make sure to line it all the way up to the sides for easy lifting of the cake after baking.
Make the Crumb Topping: Whisk together oats, brown sugar, flour, and cinnamon in mixing bowl. Add the cubed butter and cut the butter into the mixture until you achieve a crumbly, coarse mixture. Set aside for now.
In a large bowl, whisk together flour, baking soda and salt.
In a separate bowl, combine mashed bananas, sour cream, milk and vanilla extract.
In the bowl of a stand mixer ( or use an electric hand mixer), add the sugar and butter. Beat with the paddle attachment on medium high speed for about 2 minutes, until fluffy. Beat in the egg until incorporated. Scrape the sided and bottom of the mixing bowl as needed.
Reduce the mixer speed to low. Add about ⅓ of the flour mixture to the bowl. Beat to incorporate halfway, then add about half of the liquid mix. Beat until combined. Add ⅓ of the flour mixture again, incorporate halfway, then add the rest of the liquid mixture. Beat until combined. Finally, add the remaining flour, and beat the batter just until smooth. Fold in the chocolate chips.
Pour and scrape batter into the prepared loaf pan. Sprinkle all the crumb toppings evenly over the cake batter. Bake at 350 degrees F for 55-60 minutes, or until a toothpick inserted into the center of the cakes comes out clean (some bits of melted chocolate and moist crumbs may be present.
Let the pan cool on top of a wire rack for about an hour. Then lift the cake off the pan, and allow it to cool on the wire rack until its just about warm enough to slice.