Preheat oven to 350˚F. Grease and line a loaf pan with parchment paper.
In a bowl, or on a plate, mash bananas and set aside.
In a large bowl, cream together the butter and brown sugar. Add eggs, greek yogurt, and vanilla.
Add flour, baking soda, and cinnamon. With a fork, whisk flour mixture a bit, then fold into we mixture until just combined.
Mix in chopped walnuts, then fold into prepared loaf pan.
Bake for 50 to 60 minutes, or until a toothpick inserted comes out clean.
Cool, slice, and serve.