Preheat your oven to 350°F (175°C) and grease an 8x8-inch baking pan.

In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.

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

Add the eggs one at a time, mixing well, then stir in the vanilla extract, mashed ripe bananas, and sour cream.

Fold the dry ingredients into the wet ingredients.

Pour the batter into the greased baking pan and smooth the top.

In a small bowl, mix the flour, brown sugar, ground cinnamon, and salt for the streusel.

Pour the melted butter over the dry ingredients and stir until crumbly.

Sprinkle the streusel topping evenly over the batter.

Bake for 40-45 minutes or until a toothpick comes out clean.

Allow the coffee cake to cool in the pan for 15 minutes before serving.