Finely chop the chocolate amd melt over a Bain Marie. Once melted take off the heat and let cool for 5 minutes
Meanwhile, whip the cream until it thickens, but NOT as far as the soft peak stage. It should leave a ribbon trail when the beaters are lifted out of the cream
Add the egg yolks to the chocolate, one at a time, beating well between each addition with a balloon whisk
Add the cinnamon and beat in
Next, beat the thickened cream into the chocolate with the balloon whisk - work quickly to avoid the chocolate setting and the mousse turning grainy.
Whisk the egg whites to the firm peak stage and gently fold them into the chocolate mixture with a large metal spoon. Keep on folding in until no patches of egg white remain. The gentler you fold the egg white in, the lighter the mousse will be
Pour the mousse over the biscuit base, level the top if necessary, using the back of a spoon and chill for at least 6 hours (preferably overnight)