Start by blending the banana with the egg whites until you get a thick, lump-free mixture that will have slightly whipped. It will look like a smoothie. Set it aside.

In another bowl, mix the oat flour with the cocoa powder and baking powder, and sift it. Also, crush the hazelnuts as much as possible, being careful not to turn them into praline, and add them to the flour and cocoa mixture.

Put the banana mixture in the blender and gradually incorporate the flour until two-thirds are integrated, then add the soy milk and blend well. Finish the mixture by adding the sweetener and the last batch of flour.

Great! Now let's prepare it for baking. Start by preheating the oven to 340°F (170°C). Meanwhile, dampen a piece of parchment paper and wring it out to make it more manageable. Use it to line the mold you have chosen for the brownies. Remember, for brownies, it should be a square or rectangular mold at least 8 x 8 inches (20 x 20 cm).

Once the mold is prepared, spread the mixture evenly in it and decorate with the walnuts, distributing them evenly over the surface. Bake for about 30 minutes or until a knife inserted comes out clean.