FOR CRUMBLE TOPPING

1. In a medium mixing bowl, combine all-purpose flour, brown sugar, granulated sugar, salt and cinnamon. Use a whisk to thoroughly mix the ingredients.
2. Add the unsalted butter to the dry mixture. Use your hands to crumble the butter into the mixture. Set it aside.

FOR BANANA CAKE

1. Preheat the oven to 425°F (218°C). Line a baking pan with parchment paper or lightly grease it with cooking spray.
2. In a large mixing bowl, combine the milk and eggs. Use a whisk to beat them until well combined, then stir in the vanilla extract.
3. Gradually add the melted unsalted butter, mashed ripe banana, and granulated sugar to the milk/egg mixture. Stir until everything is well combined.
4. Add the all-purpose flour, baking powder, baking soda, cinnamon, and salt. Use a spatula to fold the dry ingredients into the mixture until fully combined.
5. Gently fold in the semi-sweet chocolate chips using a spatula, being careful not to overmix the batter.
6. Pour the cake batter into the lined baking pan.
7. Sprinkle the crumble topping over the cake batter
8. Bake them at 425°F (218°C) for 14 minutes, then reduce the temperature to 355°F (182°C) and bake for an additional 40 minutes or until a toothpick inserted in the center comes out clean. Enjoy your delicious banana crumble coffee cake!

Note: Please ensure to adjust the baking time and temperature according to your oven and desired level of doneness.