In a pan, whisk together the cornflour and all the other ingredients up to and including the sugar, bring to just below the boil over a medium heat while stirring. As soon as the mixture thickens, remove the pan from the heat, stir for a further 2 mins. Pour the custard into a bowl through a sieve, allow to cool, then refrigerate for approx. 15 mins.
Whisk the lemon custard until smooth, carefully fold in the whipped cream.
Sprinkle with the Toblerone.