Preheat oven to 350 degrees. Line a standard-size loaf pan with parchment paper or grease well with butter or cooking spray. Set aside.

Mix together the streusel topping until large crumbs form and set aside.

In a stand mixer with the paddle attached or with a hand mixer, beat the butter and sugar together until light and fluffy. Add the egg and vanilla and mix until well combined.

Add the bananas (I just add mine whole and let them mix for a minute or two until they're pretty mashed, but you can certainly mash them beforehand) and then the buttermilk, mixing briefly to combine. Add the remaining ingredients and mix, just until combined.

Scrape the batter into the prepared pan and sprinkle the streusel mixture on top (make sure not to go too heavy in the middle or it'll make your bread sink in the center - you can see from the photos that mine did this. It still tasted insanely good but it wasn't as good looking. . . ).

Bake for 45-60 minutes, until the edges are browned and a toothpick comes out cleanly.

Let cool before removing from pan, then slice and serve.