First make the meringue, pre-heat the oven to 120°C/gas mark 1. Clean the mixing machine and whisk in boiling water and dry with kitchen paper to remove any traces of grease, as this will affect the stiffness of the egg whites once whisked. In a mixing machine with a whisk attachment or by hand (though this will take a while), whisk the egg whites until stiff. Add the caster sugar and continue whisking until they are really stiff and shiny. Add the corn flour and vinegar and whisk again for about 45 seconds.
Spoon the mixture on to a clean baking tray, lined with silicone or greaseproof paper, into 6 or 8 round domes.
Cook in the oven for 1 ½ -2 hours or until the meringue is crisp on the outside and soft in the very middle, but don’t let the outside colour – you want it white. You may need to cook it a little longer depending on your oven; remove and leave to cool. To make the mousse: put the chocolate in a heatproof bowl over a pan of simmering water, stirring every so often until melted. Make sure you don’t get any water near the chocolate.
Remove from the heat and beat the butter into the chocolate with a whisk or spoon until smooth.
In a bowl that’s really clean – preferably washed in boiling water to remove any grease, and dried – whisk the egg whites until frothy but not stiff, using a mixer or an electric whisk on a medium high speed. Add half the sugar to the egg white and continue whisking on a low setting until the egg whites are stiff. Add the rest of the sugar and continue beating until the egg whites stiffen up even more, then mix in the egg yolks with a metal spoon.
Carefully stir half the egg mixture into the chocolate mixture – a whisk helps – then stir the rest in with a large spoon until well mixed in. Pour into a large serving dish and leave to set for a couple of hours, or overnight.
To make the chocolate sauce: put the chocolate buttons in a bowl, bring the cream and milk to the boil and pour onto the chocolate, stirring until the chocolate has melted.
To serve, hollow out the meringues by carefully pushing the centre in with the back of a spoon to make room for the crème fraîche. Fill the cavity first with the bowl