Preheat the oven to gas 1, 140°C, fan 120°C and line two baking trays with nonstick baking paper. In a clean bowl, whisk the egg whites to stiff peaks, then beat in the caster sugar, a little at a time, until thick and glossy. Sieve in the cocoa powder and fold through.
Dollop 6 tablespoons of the meringue mixture on each baking tray, leaving a small space between each one. Make a dip in the middle of each nest for the filling. Bake for 1½-2 hours until crisp on the outside and slightly chewy in the centre. Leave to cool.
Meanwhile, whip the double cream to soft peaks. Sieve in 1 tbsp icing sugar and fold through the mixture. Spoon the cream into the hollow of each nest. Decorate with mini eggs and some chocolate shavings.