Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
In a large bowl, beat the softened butter and granulated sugar together until light and fluffy, about 3 minutes.
Add the eggs one at a time, beating well after each addition, then mix in the vanilla extract.
Stir in the mashed bananas and sour cream until combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
Pour half of the batter into the prepared cake pan and spread evenly.
Drop half of the Nutella in spoonfuls over the batter. Use a knife or skewer to gently swirl the Nutella into the batter.
Pour the remaining batter over the swirled layer and spread evenly.
Drop the remaining Nutella in spoonfuls on top and swirl gently again.
Bake for 40 to 45 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and allow it to cool in the pan for 15 minutes.
Transfer the cake to a wire rack to cool completely before slicing and serving.