Preheat your oven to 350°F (175°C) and grease a 9-inch baking pan.
Mix flour, baking soda, baking powder, cinnamon, and salt in a bowl.
In another bowl, cream butter and sugar until fluffy, then beat in eggs, mashed bananas, and vanilla.
Alternate adding the dry mixture and buttermilk, mixing until the batter is smooth.
Pour the batter into the prepared pan.
For the streusel, combine flour, sugars, and cinnamon. Stir in melted butter until crumbly.
Sprinkle the streusel evenly over the cake batter.
Bake for 40 minutes or until a toothpick comes out clean. Cool before serving.