Preheat the oven to 350°F (175°C). Lightly grease an 8-inch round cake pan and line the bottom with parchment paper to ensure the cake releases cleanly.

In a medium bowl, whisk together the flour, baking powder, baking soda, salt, and cinnamon until evenly combined.

In a large mixing bowl, beat the softened butter, granulated sugar, and brown sugar using a hand mixer or stand mixer on medium speed for about 3–4 minutes until light and fluffy.

Add the eggs one at a time, beating well after each addition so the mixture becomes smooth and creamy.

Mix in the vanilla extract and mashed bananas until fully incorporated.

Add the sour cream and milk, mixing on low speed just until blended. The batter may look slightly loose at this stage, which is normal.

Gradually add the dry ingredients into the wet mixture. Mix gently on low speed until the batter is smooth and no streaks of flour remain. Avoid overmixing to keep the cake tender.

Pour the batter into the prepared cake pan and gently smooth the top.

Bake for 35–40 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 10 minutes, then transfer it to a wire rack and let it cool completely before frosting.

In a large bowl, beat the softened butter on medium speed for about 2 minutes until smooth and creamy.

Gradually add the powdered sugar, mixing on low speed so it blends smoothly into the butter.

Add the Nutella and vanilla extract, then beat on medium speed until the frosting becomes fluffy and evenly mixed.

Pour in the heavy cream and add the pinch of salt. Beat again for 1–2 minutes until the buttercream becomes silky and spreadable.

Place the completely cooled banana cake on a serving plate or cake board.

Spread a generous layer of Nutella buttercream across the top using an offset spatula.

Create gentle swirls in the frosting for a rustic bakery-style finish.

For the cleanest slices, refrigerate the cake for 20 minutes before cutting.