Heat oven to 180C/ 160C 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 surface with a wet finger, then leave for 15 mins to dry out. Bake for 15-20 mins until macaroons 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.
To make the filling, put chocolate into a heatproof bowl and gently melt over a pan of barely simmering water. Stir in the warm milk until smooth. Leave to cool and thicken a little, then use to sandwich the macaroons together