Preheat your oven to 350°F (175°C). Grease a 9x9 inch baking pan or line it with parchment paper.

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

Add the eggs, one at a time, mixing well after each addition. Mix in the mashed bananas and vanilla extract.

In a separate bowl, whisk together the flour, baking soda, and salt.

Gradually mix the dry ingredients into the wet ingredients, alternating with the sour cream, until just combined.

In a medium bowl, combine brown sugar, flour, cinnamon, and cold butter. Use your fingers or a pastry cutter to mix until crumbly.

Pour half of the banana cake batter into the prepared baking pan and then sprinkle half of the streusel topping.

Pour the remaining batter on top, and sprinkle with the remaining streusel.

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely before slicing.