Heat oven to 180°C/160°C fan/gas 4. Line a large baking sheet with baking paper. Sift the icing sugar and cocoa into a bowl, then stir in the ground almonds. Whisk the egg whites until stiff, then fold them into the dry ingredients.
Fill an icing bag fitted with a plain nozzle with the mixture (or put in a large food bag and snip off the corner). Pipe 24 small blobs, about 3cm across, onto the baking sheet, leaving a little space between each. Smooth the peak left by the piping bag with a wet finger, then leave for 15 mins to dry out. Bake for 15- 20 mins until macarons feel firm to the touch and peel easily off the paper. Cool on the paper, then peel off and store in a tin for up to 1 week. (If they last that long)!
To make the filling, put chocolate into a heatproof bowl and heat in the microwave until melted (water vapour from the traditional au bain marie may affect the chocolate). Stir in the warm milk until smooth. Leave to cool and thicken a little, then use to sandwich the macarons together. Eat as desired.