Preheat your oven to 350° F (180° C) and line an 8 or 9-inch loaf pan (6-8 cup or 2 lb) with parchment paper.
First, make the cinnamon filling. Melt the butter in a medium bowl, then use a fork to stir in the brown sugar, cinnamon, salt, and flour. Set this aside.
Make the banana bread: next, in another medium bowl, add the flour, brown sugar, baking powder, salt, and nutmeg and stir together. In a third medium bowl, mash the bananas.
In a large mixing bowl, whisk together the oil, eggs, vanilla, and mashed bananas. Once these are well combined, add the flour mixture, stirring only until it is incorporated. Avoid overmixing.
Pour 1/3 of the banana bread batter into the pan. Take 1/3 of the cinnamon filling and break it into flat pieces, then layer it over the batter, nearly covering it. Repeat this layering twice more, ending by sprinkling the last 1/3 of the cinnmon filling on top.
Bake at 350° F (180° C) for about 1 hr or until a toothpick or chopstick inserted into the center comes out clean. Serve with whipped cream cheese or peanut butter, and store at room temperature for no more than 3 days, or in the refrigerator for 4 days.
