Start by preparing the biscuits; beat the egg yolk and caster sugar together with the vanilla extract in a mixing bowl using an electric whisk. Beat for 2-3 minutes until pale and very thick. Add a few drops of red food colouring and beat again until you have a deep pink colour.
Add the flour and cornflour and gently fold into the egg and sugar mixture until even an well combined. Whisk the egg white in a separate mixing bowl until soft peaks form then whisk a third of it into the sugar, egg and flour mixture to loosen it. Fold the rest of the egg white in, working quickly and gently.
Spoon the mixture into a piping bag fitted with a small straight-sided nozzle. Grease and line a couple of large baking trays. Pipe the mixture onto the trays in lines roughly 5cm long. Use a small step spatula to smooth the lines flat so that they are about 2cm wide. Leave them to set on the trays for 10-15 minutes.
Pre-heat the oven to 180°C.
Bake the biscuits 6-8 minutes until just firm. Remove from the oven and leave to set on the trays until cool. Once the biscuits are cool, place them in a mixing bowl and crush using your hands. Spoon some of the the crushed biscuits into the base of 4 highball glasses.
Arrange the blueberries evenly on top then top with a little more crushed biscuit. Top the crushed biscuit with the redcurrants. Spoon any remaining crushed biscuit on top of the redcurrants. Slowly fill the glasses with the champagne. Garnish the tops with sprigs of redcurrants and mint leaves before serving.