Preheat your oven to 350°F (175°C). Grease an 8x8-inch baking pan or line with parchment paper.

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

In a large mixing bowl, beat the softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.

Add eggs one at a time, then mix in the vanilla extract, mashed ripe bananas, and sour cream until just combined.

Fold the dry ingredients into the wet ingredients until just combined.

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

In a small bowl, mix together the flour, brown sugar, cinnamon, and salt for the streusel topping. Stir in the melted butter until crumbly.

Evenly sprinkle the streusel topping over the batter.

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

Allow to cool in the pan for 15 minutes before slicing into squares.