Preheat your oven to 350°F. Grease a 9x5-inch loaf pan and line it with parchment paper.
In a large bowl, cream together the butter and granulated sugar until fluffy.
Beat in the eggs and vanilla until pale and fluffy.
Mix in the mashed bananas and buttermilk.
Add the flour, cinnamon, baking soda, and salt, and mix until combined.
Pour the batter into the prepared pan and sprinkle the streusel evenly over the top.
Bake for 60-70 minutes, or until a toothpick inserted into the center comes out clean with a few crumbs.
Let it cool in the pan for 30 minutes, then transfer to a rack to cool completely or enjoy it warm.
For the best flavor, let it cool completely before digging in!
Store any leftovers in an airtight container for up to three days.