For the banana muffins, preheat the oven to 190 °C. Peel banana, mash with a fork and sprinkle with lemon juice.
Mix and sift flour, baking powder and cocoa powder. Separate two eggs. Whisk one egg and two yolks.
Add sugar and cinnamon, put everything in food processor and whisk until fluffy. Stir in milk, butter and banana and sift in flour mixture.
Beat egg whites until stiff and gently fold into batter. Grease muffin tray and dust with flour, then pour in batter.
Bake for half an hour (test with chopsticks).