Whilst the chocolate is melting, whisk the egg whites and lemon juice in a large, clean bowl until they form soft peaks. The lemon juice will stabilise the egg whites, make them easier to work with. Add the sugar and continue to whisk until firm peaks form when the whisk is removed. In another bowl, whisk the cocoa powder and the fresh cream together to soft peaks and put aside. When the chocolate has cooled down slightly, whisk in half of the egg whites quickly, stirring vigorously until thick and well combined. Fold in the remaining egg whites into the chocolate mixture, using a spatula, until all of the egg white has been completely incorporated, followed by the whipped chocolate fresh cream. Quickly spoon the mousse mixture into four Martini glasses. Chill in the fridge for 2-3 hours, or until set. Decorate with chocolate straws or shavings before serving.