Combine the ground nuts, cocoa, lemon peel, salt and vanilla in a small bowl.
In a large bowl beat the egg whites until they foam and thicken slightly. Sprinkle the sugar over them and continue to beat until whites form stiff unwavering peaks on the beater when lifted out of the bowl. Gently fold the nut mixture into the whites until thoroughly combined.
Coat a large baking tray with 2 teaspoons softened butter. Drop the dough by tablespoons onto the prepared baking tray, spacing them about 3cm apart. Let the biscuits rest at room temperature for 1 hour before baking.
Preheat the oven to 150 degrees C. Place the biscuits in the middle of the oven; bake for 30 minutes or until they are firm. With a spatula carefully transfer the biscuits to a rack to cool.