Preheat the oven to 110°C (90°C fans) and line three baking sheets with parchment paper. Draw a circle on each piece of parchment using a 20cm cake pan as a pattern, then flip it over, so the design is below.
Whisk the egg whites in a clean, grease-free dish with an electric mixer until firm peaks form. Pour the caster sugar gradually and whisk until the meringue is firm and glossy. Sift the icing sugar and cocoa powder over the meringue and fold in as gently as possible.
Spread the meringue onto the baking parchment using the drawn templates, starting in the middle and working outwards, piping in a spiral from the center outwards. Pipe long meringue strips alongside the discs onto the prepared trays using the remaining meringue. Bake for about 1 hour and 40 minutes, or until firm and crisp. Let the meringues cool for two hours before removing them from the oven.
To prepare the mousse, melt the chocolate in a large heatproof bowl positioned over a pan of gently boiling water, being careful not to let the bottom of the dish contact the water.
Take the pan from the heat and set it aside to cool. Whip the cream to soft peaks, then pour it into the chocolate dish and incorporate. The final mousse should be pretty thick and able to keep its form, but not so thick that it is difficult to distribute. If the mousse is too thick and seems overwhipped, add more cream or milk and mix to soften it up.
To build the cake. A small quantity of the mousse should be applied to the first meringue disc and glued to a serving plate or cake board.
Spread approximately a third of the mousse over the meringue, then top with another meringue and repeat.
Finish with the last meringue and then the remaining third of the mousse on the top and sides of the cake.
Cut the meringue strips into pieces using a serrated knife to complete the decorating. Press them gently all over the cake and sprinkle with icing sugar or cocoa powder. You may serve the cake right away or freeze it for a few hours to soften the meringue somewhat. If you decide to freeze the cake, let it defrost in the fridge for a few hours before serving.