Preheat your oven to 325°F (160°C) and line an 8x8 inch metal pan with parchment paper.

In a medium bowl, combine brown sugar, ground cinnamon, all-purpose flour, and kosher salt to make the filling. Set aside.

In a separate bowl, mix softened unsalted butter, brown sugar, ground cinnamon, all-purpose flour, and kosher salt to make the crumble topping. Set aside.

Mash the ripe bananas in a large mixing bowl. Add granulated sugar, egg, and melted butter, mixing until creamy.

Stir in all-purpose flour, ground cinnamon, kosher salt, baking soda, and baking powder until just combined.

Whisk together milk, apple cider vinegar, and vanilla extract in a separate bowl. Pour this into the banana mixture, mixing until smooth.

Pour half of the batter into the prepared pan. Sprinkle the filling mixture evenly over the batter, then pour the remaining batter on top.

Sprinkle the crumble topping evenly over the cake batter.

Bake for 45-50 minutes until golden and a toothpick inserted comes out clean.

Allow the cake to cool completely in the pan before serving. Optionally dust with powdered sugar before serving.