Preheat the oven to 350°F (175°C). Grease and flour a 9×5-inch loaf pan.
In a medium bowl, whisk together 1 1/2 cups flour, baking soda, salt, and 1/2 teaspoon cinnamon. Set aside.
In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
In a separate small bowl, combine mashed bananas, sour cream, and milk.
Add the banana mixture to the butter mixture and mix until combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
Pour the batter into the prepared loaf pan and smooth the top.
To make the coffee cake topping, combine brown sugar, 1/2 cup flour, and 1/2 teaspoon cinnamon in a small bowl.
Cut in the cold butter pieces using a pastry cutter or your fingers until the mixture resembles coarse crumbs.
Sprinkle the topping evenly over the banana bread batter.
Bake for 55-60 minutes, or until a toothpick inserted into the center comes out clean.
Remove from the oven and allow the bread to cool in the pan for 10 minutes.
Transfer the bread to a wire rack to cool completely before slicing.