Preheat oven to 350 degrees. Grease a 9x5-inch loaf pan and line it with parchment paper. (The parchment paper is not optional! The banana bread won't bake properly without it.) Set aside.
In a large bowl cream the butter and granulated sugar together with an electric mixer.
Add in the eggs and vanilla and mix for one minute until pale and fluffy.
Then mix in the mashed banana and buttermilk.
Add in the flour, cinnamon, baking soda and salt and mixt until thoroughly combined
Pour the batter into the prepared loaf pan. Sprinkle the streusel over the top, making sure to cover the whole surface.
Bake the banana bread for 60-70 minutes, or until a toothpick comes out clean from the center with only a few moist crumbs.
Allow the banana bread to cool in the pan for 30 minutes, or until cool enough to handle.
Transfer the bread to a cooling rack to completely cool, or enjoy warm. The banana bread does taste best when it is completely cooled though!
Store the leftovers in an airtight container for up to three days.