For the salted caramel sauce, place the sugar, two tablespoons water and golden syrup in a saucepan and bring to a boil, stirring occasionally. When the sugar has turned a rich amber colour, remove from the heat and stir in the butter, cream and salt. Pour into a bowl, cover the surface with clingfilm and cool completely.
For the mousse, melt the chocolate and butter in a bowl set over a pan of simmering water - ensuring that the bottom of the bowl does not touch the water. Set aside to cool a little. Then whisk the egg yolks into the cooled chocolate mixture.
In a separate bowl, whisk the egg whites to a light foam then add the caster sugar and whisk again until soft peaks form when the whisk is removed. Stir a third of the egg whites into the chocolate mixture. Then carefully fold in the remaining egg whites until well incorporated.
Pour a tablespoon of salted caramel sauce into the bottom of each serving glass. Top with one-third of the mousse mixture. Drizzle over enough sauce to cover the mousse. Repeat this layering with mousse and caramel, finishing with a final layer of mousse. Refrigerate for an hour before serving. If chilled for longer, remove 30 minutes before serving. Finish with a dollop of whipped cream.