Make the banana cake layers first. Preheat the oven to 350°F/175°C and grease and line 3, 8-inch cake pans. Set aside.
Mash 3 large bananas with either a fork or a mixer and stir together with 2 tsp of lemon juice. This prevents the bananas from browning. Set aside.
In a large bowl, whisk 3 cups flour, 1 tsp baking powder, 1 tsp baking soda, and 1/2 tsp salt together. Set aside.
Use a hand mixer or stand mixer fitted with a whisk attachment to beat 1 cup of butter, 1 cup of granulated sugar, and 2/3 cup of brown sugar together. Mix on a high speed for about 3 minutes. The mixture should get lighter in color as air is whipped into it. Use a spatula to scrape the sides and bottom of the bowl as needed.
Mix in 3 large eggs and 2 tsp of vanilla extract on a medium speed.
Mix in half of the dry ingredients on a low speed until just combined.
Add in 1 1/2 cups of buttermilk and mix until incorporated, then mix in the remaining dry ingredients on a low speed. Be careful not to over-mix the batter once you've added in the flour. The batter will be on the thick side.
Mix in the mashed bananas on a medium speed until just combined.
Divide the batter evenly between the prepared pans. If desired, add small spoonfuls or drizzle warm Nutella (heated in the microwave for 30 seconds) on top of the banana cake batter.
Use a small offset spatula or butter knife to drag horizontal and vertical lines through the batter to create a swirled pattern with the Nutella.
Bake for 31-35 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. If you don't add the Nutella swirl, reduce the bake time by 3 minutes.
Remove the cakes from the oven and set on a wire rack. To help flatten the cake layers without ruining the Nutella swirl on top, use a slightly smaller pan (like a 7" or 6" pan) to gently press down on the cake layer right after they come out of the oven.
Run an offset spatula around the edge of the pan to help release the cake layers from the pan.
Let the cakes cool fully in the pan, then carefully remove them.
Vanilla and Nutella Buttercream
As the cake layers bake and cool, make the Nutella buttercream.
Beat 2 cups of unsalted butter on a medium-high speed for a minute with a paddle attachment or hand mixer until smooth and lighter in color.
Mix in 2 tsp vanilla extract and 1/2 tsp salt on a low speed.
Slowly mix in 7 cups powdered sugar on a low speed. Scrape the sides and bottom of the bowl as need with a rubber spatula.
Add 1/3 cup of heavy cream halfway through adding in the powdered sugar to make it easier to mix.
Beat on low until the ingredients are fully incorporated and the desired consistency is reached.
If the frosting is too thick, add an extra Tbsp of heavy cream. If it's too thin, add in another 1/4 cup of powdered sugar.
Stir by hand with a rubber spatula to get the frosting SUPER smooth.
Wait to mix in the Nutella until after the cake has been crumb coated. Cover flush with plastic wrap to prevent crusting and set aside.
Banana Nutella Cake Assembly
Stack and frost the cake layers on a 10-inch, grease-proof, cardboard cake round or flat plate.
Spread a tablespoon of frosting in the center of the board to help keep the first cake layer in place.
Spread a thick, even layer of vanilla buttercream on top of the cake layer, then drizzle and smooth 1/2 cup of warm Nutella on top of the frosting. If desired, top with fresh banana slices. Repeat with remaining layers.
Next, cover the cake in a thin layer of frosting. Make sure you get enough frosting between the cake layers to fill any gaps and keep the filling place.
Smooth using a large offset spatula and bench scraper, then chill in the freezer (5 mins) or fridge (30 mins) until the frosting is firm to the touch. This helps lock in the filling and makes it easier to get a perfect, second coat of frosting.
As the cake chills, mix 2/3 cup of Nutella into the remaining buttercream. If the frosting seems too thick, add an additional Tablespoon or two of heavy cream.
Remove the cake from the fridge or freezer, then add a second, thicker layer of Nutella frosting around the cake. Smooth using a bench scraper. You can see a full tutorial on how to frost a cake with smooth sides here.
Top the cake with a circle of drizzled, warm Nutella, fresh bananas or banana chips, and some chocolate chips.
Video
Notes
If you want to make this recipe in a different sized pan, please see the "Making This Banana Nutella Cake in Different Sizes" section above.
Tips for Making the Best Banana Nutella Layer Cake
Ingredients at room temp mix together better! Set out any cold ingredients ahead of time.
Take the time to properly cream together the butter and sugar. The mixture should look lighter in color after being mixed on high for 3 minutes.
Properly measure the flour (spoon into the cup measure, then level). Or better yet, use a kitchen scale to measure your dry ingredients.
Use a scale to weigh your cake pans as you fill them. Make sure each pan has the same amount of batter so that cake layers bake to the same height and bake more evenly.
Bang the cake pans on the counter before putting them in the oven. This brings any air bubbles that are trapped in the batter to the surface.
To help flatten the cake layers without ruining the Nutella swirl on top, use a slightly smaller pan (like a 7" or 6" pan) to gently press down on the cake layers right after they come out of the oven. This will make the layers so much easier to fill and stack.
Chill the cake layers in the freezer for about 20 minutes before assembling the cake. It makes it so much easier to stack and frost them!
Making this Banana Nutella Cake in Advance and Storage Tips
Make the cake layers in advance and freeze them. It breaks the process up and makes it more approachable.
Make the frosting ahead of time or save any leftover frosting! It can be stored in an airtight container in the fridge for up to a month.
This frosting can also be stored in the freezer for up to 3 months! Just be sure to give it a good mix once it thaws to make it nice and smooth again.
A frosted cake can last in the fridge for up to 3 days. The buttercream locks in the moisture and keeps the cake fresh and delicious!
I don't recommend freezing this cake, it gives the bananas a strange texture.
Note: If you don't plan to eat this cake immediately, the bananas on the top of the cake will brown! To prevent this, you can either brush the bananas with lemon juice to prevent browning or use banana chips.