Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.

In a mixing bowl, mash the bananas until smooth with a fork or potato masher.

Add softened butter and sugar to the mashed bananas; blend until creamy.

Incorporate the eggs and vanilla extract into the banana mixture and beat until smooth.

Whisk together flour, salt, and baking powder in a separate bowl.

Gradually add the dry mixture to the banana mixture, alternating with milk, until just combined.

Prepare the crumb topping by mixing brown sugar, flour, and cubed cold butter until resembling coarse crumbs.

Pour half of the banana batter into the prepared pan, sprinkle with half of the crumb topping, followed by the remaining batter and crumb topping.

Bake for approximately 50 minutes or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for 10-15 minutes before slicing.