Heat the oven to 350°F (175°C). Grease a 9 x 5-inch loaf pan and line it with parchment paper, leaving enough overhang to lift the baked loaf out easily.

Whisk the flour, baking soda, salt, and cinnamon together in a medium bowl. In a separate large bowl, beat the softened butter and brown sugar for about 2 minutes, until smooth and creamy.

Beat the eggs into the butter mixture one at a time. Mix in the yogurt or sour cream, mashed bananas, and vanilla until evenly combined.

Add the dry ingredients to the banana mixture and stir just until no dry flour remains. Do not overmix, because excessive stirring can make the finished loaf dense.

Spoon about half of the batter into the prepared pan. Warm the Nutella for 10 to 20 seconds in the microwave if needed, then spread about half over the batter in spoonfuls.

Add the remaining batter and top with the remaining Nutella. Draw a butter knife through the batter several times to make a swirl, taking care not to blend the Nutella completely into the banana batter.

Bake for 60 to 65 minutes, until a toothpick inserted into the banana-bread portion comes out with a few moist crumbs but no wet batter. If the top browns too quickly, loosely cover the loaf with foil during the last 20 to 25 minutes.

Cool the loaf in the pan for 10 to 15 minutes, then use the parchment overhang to transfer it to a wire rack. Let it cool for at least another 20 to 30 minutes before slicing.