Preheat your oven to 350°F (175°C). Grease a 9-inch cake pan and set it aside.
In a bowl, mash bananas until smooth and creamy.
Cream butter and sugar in a separate bowl until fluffy, then add eggs and vanilla, mixing well.
Combine flour, baking powder, baking soda, and salt in another bowl.
Gradually alternate adding dry ingredients and milk to the wet mixture. Stir until smooth.
For the streusel, mix flour, brown sugar, cinnamon, and cold butter until crumbly.
Pour half the batter into the prepared pan, then sprinkle half the streusel on top.
Add the remaining batter and finish with the rest of the streusel.
Bake for 35 minutes or until a toothpick comes out clean.
Cool slightly before slicing and serving.