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1
Preheat the oven to 110C (90C fan oven) mark 1/4 and line three baking sheets with baking parchment.
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2
Using a 20cm cake tin as a template, draw a circle on each piece of parchment, then turn it over so that the drawing is underneath.
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3
Put the egg whites into a clean, grease-free bowl and, using an electric mixer, whisk until they form stiff peaks.
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4
Slowly pour in the caster sugar and whisk until the meringue is stiff and glossy.
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5
Sift the icing sugar and cocoa powder over the meringue and gently fold together, being as gentle as possible.
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6
Spoon the meringue into a piping bag fitted with a 1cm wide plain piping tip and pipe three discs on the baking parchment using the drawn templates, piping in a spiral starting at the centre and working outwards.
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7
Using the remaining meringue, pipe long strips onto the prepared trays alongside the discs.
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8
Bake for about 1 hour 40 minutes or until firm and crisp.
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9
Turn off the oven and allow the meringues to cool in the oven for 2 hours.
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10
To make the mousse, melt the chocolate in a large heatproof bowl set over a pan of gently simmering water, making sure the base of the bowl doesnt touch the water.
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11
Remove from the heat and leave to cool.
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12
Whisk the cream to soft peaks then pour it into the bowl with the chocolate and whisk to combine.
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13
The resulting mousse should be fairly thick and be able to hold its shape, but not so thick it cant be spread easily.
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14
If it is too thick and looks overwhipped, pour in a little extra cream or milk and stir to loosen the mousse.
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15
To assemble the cake, use a little of the mousse to stick the first meringue disc to a cardboard cake board or serving plate.
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16
Spread about a third of the mousse over the meringue and then add another meringue and repeat the process.
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17
Top with the final meringue and then coat the top and sides of the cake with the remaining third of the mousse.
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18
To finish the decoration, use a serrated knife to gently cut the meringue strips into pieces.
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19
Lightly press these all over the cake and lightly dust with either icing sugar or cocoa powder.
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20
You can either serve the cake now or you can freeze it for a few hours; this will soften the meringue slightly.
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21
If you do freeze the cake, thaw it in the fridge for a few hours before serving.
