Line 2 baking sheets with nonstick baking paper. Put the icing sugar in a food processor with the ground almonds and cocoa powder and blend to a very fine consistency.
Put the egg whites in a thoroughly clean bowl and whisk until peaking. Gradually whisk in the caster sugar, a tablespoonful at a time and whisking well after each addition, until thick and very glossy. Add the almond mixture to the bowl and use a metal spoon to stir the ingredients together to combine.
Place in a piping bag fitted with a 1 cm (½ inch) plain nozzle and pipe 24 x 3 cm (1¼ inch) rounds onto the baking sheets. Tap the baking sheet firmly to smooth the surfaces of the macaroons slightly, then leave to stand for 30 minutes. Bake in a preheated oven, 160°C (325°F), Gas Mark 3, for about 15 minutes or until the surfaces feel crisp. Leave to cool before carefully peeling away the paper.
Heat the cream in a small saucepan until bubbling up around the edges but not boiling. Remove from the heat and stir in the chopped chocolate. Leave the chocolate to melt, stirring frequently, until smooth. Once the chocolate is cool and thickened enough to hold its shape, use to sandwich the macaroons together in pairs. Keep in a cool place until ready to serve.