1. Preheat oven to 350°F (175°C). Grease two 8×4-inch loaf pans or line with parchment. In a small bowl, mash bananas with brown sugar, vanilla, and 3/4 tsp cinnamon until mostly smooth (leave some chunks for texture). Set aside. In a large bowl, beat softened butter and granulated sugar until light and fluffy, about 2 minutes. Add eggs one at a time, beating well after each. In a separate bowl, whisk flour, baking soda, and salt. Gradually add dry ingredients to wet, mixing just until combined. Fold in sour cream, then banana mixture—avoid overmixing for tenderness. For the crumb topping: In a medium bowl, combine cinnamon, flour, and brown sugar. Cut in cold butter using a pastry blender or fork until crumbly (pea-sized pieces). Divide batter between pans, then generously pile topping over each (use all of it!). Bake 35-40 minutes, until a toothpick inserted in the center comes out with moist crumbs (not wet batter). Cool in pans 10 minutes, then transfer to a wire rack. Slice once fully cooled.