In a mixing bowl, mash 1 banana into small pieces, then add the sugar, vanilla, melted butter and egg. Beat with a mixer until smooth and well combined.
Add the flour and baking powder and mix until you get a thick, even batter.
Finely chop the almonds, hazelnuts and chocolate, then fold them into the batter with a spatula.
Line a loaf pan with parchment paper and pour in the batter, smoothing the top.
Slice the second banana in half lengthwise and place the halves on top of the batter, cut side up.
Bake at 350°F (180°C) for 50–60 minutes, or until a toothpick inserted in the center comes out clean.
Notes
Use very ripe bananas with spotted peels, as they provide more sweetness and flavor, allowing you to slightly reduce the amount of sugar if they are very sweet.
Take the butter and egg out of the fridge 15–20 minutes in advance so they are not too cold; this helps the batter mix more evenly and the loaf rise more uniformly.
Do not overmix after adding the flour and baking powder—just gently incorporate them; if you overbeat the batter, the loaf will turn out dense and heavy.
Check the banana bread after 50 minutes using the toothpick test in the center of the pan; if it comes out with just a few moist crumbs, you can remove it so it stays slightly moist inside.
Let the banana bread cool for at least 15–20 minutes in the pan, then transfer it to a wire rack; if you slice it while hot, it will crumble and you won’t get clean slices.