Preheat oven to 350 degrees F. Grease a 9x5 loaf pan with butter. Set aside.
Dry ingredients: In a bowl, whisk the flour, baking soda, salt, and ground cinnamon until evenly combined. Set aside.
With a mixer on high speed, cream the softened butter with the brown sugar and granulated sugar until light and creamy, about 2 minutes. Add the eggs, one at a time, mixing on medium speed. Add the mashed bananas, Greek yogurt and vanilla extract, and mix on medium speed until evenly combined.
Gradually add in the dry ingredients to the mixture, combining by hand using a rubber spatula (or using a mixer on low speed). Do not overmix. Set aside.
Prepare the Nutella: Pour the chocolate hazelnut spread (Nutella) into a microwave safe bowl. Microwave for about 15 to seconds, or until it becomes drizzle-able. Stir with a spoon after 10 secs if needed to loosen it up. Note: Do not microwave for more than 25-30 seconds, or it will burn.
Pour half of the banana bread batter into the prepared loaf pan. Smooth it over, and drizzle half of the Nutella over the batter. Swirl it using a knife. Then, pour the remaining banana bread batter on top, smooth it over, and drizzle the remaining Nutella on top. Swirl with a knife, forming designs if desired. Tip: try not to accumulate the Nutella in the center so it's not too heavy, and so it can rise properly.
Bake in the middle rack for 55-60 minutes, or until a toothpick inserted in the middle comes out clean. After about 30 minutes into the baking time, loosely cover with aluminum foil to prevent it from browning too fast.
Remove from oven and let it cool for 20 minutes before removing from pan. Then, carefully remove from the pan and let it cool a little bit more before serving. Enjoy!