Sift the icing sugar and cocoa into a bowl, then add the almonds.
Whisk the egg whites until stiff peaks form, then add to the dry ingredients and fold in. Don’t worry about being too gently, it’s important to get a smoothly flowing ‘lava-like’ mixture.
Line baking trays with parchment. Fill an icing bag with a plain nozzle and pipe blobs about 4cm across – makes about 36.
Leave for about 20 minutes until a skin has formed. Preheat oven to 160C.
Bake for 15 minutes until the macaroons are firm to the touch. Cool for a few moments then use a palette knife to transfer to a wire rack.
Melt the chocolate together and let cool for a couple of minutes; warm the milk and stir in. Keep stirring even if it looks like it has seized. Once smooth, leave to cool for about 10 minutes until it has started to thicken.
Using a piping bag, pipe a blob onto half of the macaroon shells; sandwich together. Melt and drizzle with extra dark chocolate if desired.