Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan or line it with parchment paper.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy.

Add the mashed bananas, eggs, and vanilla extract, mixing until well combined.

In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.

Gradually add the dry ingredients to the banana mixture, alternating with the sour cream, and mix until just combined.

To make the crumb topping, combine the flour, brown sugar, melted butter, and cinnamon in a small bowl. Mix until crumbly.

Pour the banana cake batter into the prepared pan and sprinkle the crumb topping evenly over the batter.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.