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, cream the softened unsalted butter and granulated sugar together until light and fluffy, about 3-4 minutes.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.
Mix in the mashed ripe bananas and sour cream until just combined.
Gradually add the dry ingredients to the wet ingredients, folding gently until just combined. Do not overmix.
Pour half of the batter into the prepared cake pan and spread evenly.
Drop spoonfuls of Nutella over the batter and gently swirl it with a knife or skewer to create a marbled effect.
Pour the remaining batter over the Nutella layer and spread evenly.
Add more spoonfuls of Nutella on top and swirl again gently.
Bake in the preheated oven for 45-50 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Remove the cake from the oven and let it cool in the pan for 10 minutes.
Transfer the cake to a wire rack to cool completely before slicing and serving.