Preheat the oven to 400 degrees F and line a baking sheet with parchment or preferably silicon pad.
Mix egg whites (unbeaten) with the ground almonds, cocoa and powdered sugar until you have a sticky but cohesive mixture.
Fill a large bowl with cold water and dip your hands in it to wet them before rolling the mixture into little balls the size of small walnuts. You will probably have to redunk your hands to keep wetting them as you go.
Arrange the macaroon-balls on the lined baking tray and put in the oven to bake for 11 minutes. It's hard to tell when they're ready, as they will seem squishy but they harden up a little as they cool, and anyway, they should be damp within; that's what makes them chewy, so don't worry that the macaroons look sticky underneath.
Recipe courtesy Nigella Lawson, 2007