1. Preheat oven to 350 degrees F. Grease a 9x13 pan and set aside.
2. In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
3. In a large bowl, cream together the butter and sugar until creamy and smooth, about 3 minutes. Beat in the eggs, one at a time, then stir in the yogurt and vanilla extract. Mix in the mashed bananas.
4. Gradually add the dry ingredients and mix until flour disappears. Spread cake batter evenly into the prepared pan. Swirl the Nutella into the cake using a knife of toothpick.
5. Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean. Allow cake to cool completely.
6. While the cake is cooling, make the Nutella frosting. In the bowl of a stand mixer, beat the butter and Nutella on high until smooth. Slowly add the powdered sugar, 1/4 cup at a time. Beat on high until powdered sugar is incorporated. Add the milk and hazelnut extract, if using. Beat until frosting is smooth and desired consistency. If you need to add more milk, add a little bit at a time.
7. Frost the cake with Nutella frosting. Cut into squares and serve. I added sprinkles for fun!