Preheat the oven to 150°/Gas Mark 2. Line a heavy baking tray with baking parchment and brush lightly with vegetable oil. Next to you have a small bowl of water
Put the egg whites, icing sugar, flaked almonds and orange zest in a bowl and gently mix them together. Dip your hand in the bowl of water and pick up portions of the mix to make little mounds on the lined tray, well spaced apart. Dip a fork in the water and flatten each biscuit very thinly. Try to make them as thin as possible without creating too many gaps between the almond flakes. They should be about 8cm in diameter.
Place the baking tray in the oven and bake for about 12 minutes, until the biscuits are golden brown. Check underneath one biscuit to make sure they are cooked through.
Allow to cool, then gently using a palette knife, remove the biscuits from the baking sheet.
Brush one side with melted dark chocolate, to justify the name and give it a more substantial texture.
Makes around 20 and should be stored in a sealed jar