Preheat the oven to 350°F (175°C). Grease and flour an 8-inch (20 cm) round cake pan.
In a medium bowl, whisk together the all-purpose flour, baking powder, 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. Stir in the vanilla extract.
Mix in the mashed bananas and buttermilk until combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Do not overmix.
Pour half of the banana cake batter into the prepared cake pan and spread evenly.
Warm the Nutella slightly in the microwave for about 15 seconds to make it easier to swirl.
Drop spoonfuls of Nutella evenly over the batter in the pan.
Pour the remaining banana cake batter over the Nutella layer and spread evenly.
Using a knife or skewer, gently swirl the Nutella through the batter to create a marbled effect.
Bake in the preheated oven 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 10 minutes.
Transfer the cake to a wire rack to cool completely before slicing and serving.