In a large bowl, whisk together flour, baking powder, cinnamon and salt. Set aside.
In your mixer, add the butter and sugar and cream together until well-combined
Add eggs, vanilla and mashed bananas. Combine well.
Add in half of the dry mixture and half the buttermilk. Mix well.
Add the other half and rest of buttermilk. Mix well.
Grease a 9x13 pan and cover it in parchment paper. Pour cake batter into pan. Set aside.
Streusel: In a large bowl, add the cold butter cubes, flour, sugars and cinnamon. Use your hands and combine well. Mush together well until it resembles a crumb mixture.
Add streusel to the top of the cake batter
Bake at 350 degrees for 50-60 minutes making sure a toothpick comes out clean