- Preheat the oven to 350°F (175°C). Grease a 9×13-inch pan or two 8-inch round pans.
- In a large bowl, mash the bananas until smooth. Add the cake mix, eggs, oil, buttermilk, and vanilla. Beat until just combined—don’t overmix.
- Pour the batter into the prepared pan(s) and bake for 25–32 minutes, or until a toothpick inserted in the center comes out clean. Cool completely.
- While the cake cools, make the frosting: beat the butter until creamy. Add cocoa powder, powdered sugar, heavy cream, Nutella, and vanilla. Beat until light and fluffy, adding more cream if needed for a smooth consistency.
- Spread the frosting evenly over the cooled cake, reserving about 1 cup to pipe a border if desired. Use a large shell tip to pipe the edges.
- Finish by adding sprinkles and mini chocolate chips to the border, if desired. Slice and serve!
*Note- if you do not want to pipe a border, you can cut the frosting recipe in half.