Preheat oven at to 325. Grease light colored loaf pan with nonstick cooking spray and set aside
Start by making the crumble; Combine the flour, cinnamon, and sugars in a bowl, and mix in the melted butter. The mixture should resemble wet sand. Place the bowl in the refrigerator to chill while making the rest of the loaf is made
In a large bowl, mix oil, butter, and sugar until well combined, stir in eggs, vanilla and bananas until smooth
Gently stir dry ingredients and buttermilk until a uniform batter is formed. Scrape sides to ensure no lumps are present. Stir in ½ cup walnuts. Pour into prepared pan
Mix the cinnamon sugar together, and pour over the batter by taking a toothpick making swirls
Before adding the loaf pan into the oven add the crumble on top of the banana batter
Bake for 50-60 minutes in the lower half of the oven, until it is completely baked. Remove from the oven and cool for at least 15 minutes before removing from the pan. Slice and serve
