Make the custard: Heat the milk in a medium pan with the lemon zest until it is almost boiling. In a large bowl, whisk the egg yolks together with the cornflour, sugar and essence. When the milk is hot, pour it onto the egg mixture, whisking all the time. Rinse out the saucepan (or the custard will stick), then return the custard to the pan.
Gently heat the custard over a gentle heat, stirring with a wooden spoon until the mixture begins to thicken. Gently simmer for 1 minute, stirring all the time. Remove from the heat, add 50g of the white chocolate and continue to stir until it has melted into the custard. Set aside to cool slightly.
Reserve 15 blueberries for decoration.
Cut the cake into 2cm cubes and scatter over the base of a large glass serving bowl. Mix the lemon juice and sherry together and scatter over the sponge. Scatter over the remaining blueberries, then pour over slightly cooled custard. Chill until set.
In a large bowl, whip the cream until it forms soft peaks. Heap on top of the custard. Chop the remaining chocolate and scatter over the cream with the reserved blueberries.
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