Preheat the oven the 180°C/350°F, without the fan option, and arrange a baking tray onto the middle shelf.
Transfer the bananas in a large bowl and mash them with a fork until reaching a creamy texture. You can also use a blender or a stick blender.
Pour slightly warmed chocolate spread into the bowl, followed by the cocoa powder.
Mix all the ingredients with a spatula until fully combined.
Spray with baking oil an 8-inch square cake pan, pour in the batter, and level the top using a spatula. At this point you cna sprinkle some fleur de sel (sea salt flakes) on top if you like.
Bake in the oven (without the fan option) for about 35-40 minutes, then check if the surface looks cooked through, then open the oven and check with a toothpick inserted in the center of the batter if it's done.
If the toothpick comes out almost clean, the brownies are ready and you can safely remove the pan from the oven.
Allow cooling completely removing from the pan, then cut into 9 squares and serve. For best texture, refrigerate them for at least 30 minutes before serving.