Preheat the oven to 150°C (130°C Fan, Gas Mark 2). Line 2 baking trays with baking parchment.
Put the icing sugar, almonds and cocoa powder in a blender or food processor and blend for a few seconds until finely ground. Set aside.
Make up the Egg White Powder according to the packet instructions, then whisk until very stiff. Whisk in the caster sugar.
Sift the ground ingredients on top and carefully mix into the egg white until well blended. The mixture should be soft yet pipeable.
Spoon the macaroon mixture into a piping bag fitted with a 1cm (½ inch) plain round nozzle. Pipe 5cm (2inch) diameter rounds on to the prepared baking trays – you should be able to make 18 rounds. Tap the trays gently on the work surface to smooth and settle the tops. Leave the piped mixture at room temperature for about 30 minutes to form a slight skin on the surface – the outside should be dry, not sticky when touched lightly with your finger.
Bake in the oven for 20-25 minutes until firm to the touch and slightly risen. Leave to cool on the baking trays for 10 minutes then transfer to a wire rack to cool completely.
For the filling, break the Extra Dark Chocolate into a small heatproof bowl. Put the bowl over a saucepan of barely simmering water and leave to melt. Remove from the water and leave to cool for about 20 minutes until slightly warm.
Whilst whisking the chocolate, gradually pour in the cream, whisking until thickly whipped and glossy.
Spread a small dollop of cream over one half of the macaroons and sandwich together. Leave in a cool place for several minutes to firm before serving. Serve as soon as possible after filling.