Preheat your oven to 350°F (175°C). This ensures that your cake bakes evenly and rises beautifully.
Grease your baking pan with a bit of oil or butter, and line the bottom with parchment paper for easy removal.
In a large mixing bowl, combine the mashed bananas, light brown sugar, vanilla, egg, vegetable oil, and buttermilk. Use a whisk to blend everything together until smooth and well incorporated.
In another bowl, sift together the Dutch process cocoa powder, all-purpose flour, baking soda, and a pinch of salt. This step helps to eliminate lumps and evenly distribute the baking soda.
Gradually add the dry ingredients to the wet mixture, stirring gently with a rubber spatula until just combined. Be careful not to overmix; a few lumps are perfectly fine.
Gently fold in the milk chocolate pieces and chopped hazelnuts until evenly distributed throughout the batter.
Pour the batter into the prepared baking pan, spreading it evenly with your spatula.
Place the pan in the preheated oven and bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely. Slice and serve as is, or dress it up with your favorite frosting or a scoop of ice cream!