In a large bowl, cream together the butter and sugars (not the sugar for the crumb topping though, we'll do that last). Stir in the eggs, vanilla, mashed bananas, and sour cream or greek yogurt. In a second bowl, whisk together the flour, cinnamon, baking powder, salt, and baking soda. Combine the dry ingredients with the wet until a thick batter forms.
In a medium bowl, make the crumb topping by combining the sugar, flour, and cinnamon. Cut the cold butter into small cubes and add to the bowl. Use a pastry blender or your clean hands to combine. The mixture should be crumbly, but if you press it together in the palm of your hand it should hold together before crumbling again. Similar to a pie crust consistency, but a little dryer.
In a bundt pan that has been lightly oiled or buttered add the cake batter. Sprinkle the crumb topping over the top. Bake at 350°F for 35-38 minutes. Insert a toothpick into the cake to test if it's done (it shouldn't have gooey, unbaked batter stuck to it when you pull it out). Allow to cool in the pan for 10-15 minutes before removing.
If you want to bake this in a square baking pan, like brownies, you can. Simply aim for 30-35 minutes of bake time instead. Again, it's best to test with a toothpick as all ovens can be slightly different (especially in high altitude).