- Prepare the crumb topping: In a medium bowl, combine flour, brown sugar, cinnamon, and salt. Add softened butter and mix with a fork or pastry cutter until the mixture resembles coarse crumbs. Set aside.
- Prepare the cake: Preheat your oven to 350°F (175°C) and grease or line a 9-inch square or round cake pan.
- In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.
- In a large bowl, beat the softened butter and granulated sugar with an electric mixer until light and fluffy, about 3-4 minutes.
- Add the eggs one at a time, mixing well after each. Stir in the mashed bananas and vanilla extract.
- Gradually add the dry ingredients to the wet ingredients in batches, alternating with the sour cream or yogurt. Begin and end with the dry ingredients. Mix until just combined.
- Pour the batter into the prepared pan and smooth the top evenly.
- Assemble and bake: Sprinkle the crumb topping evenly over the cake batter. Bake for 35-40 minutes or until a toothpick inserted into the center comes out clean. If the topping begins to brown too much, cover the cake loosely with aluminum foil.
- Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.
Serve: Slice and serve while warm or at room temperature. This cake is perfect with a cup of coffee or tea!