In a mixing bowl, combine the egg whites, caster sugar, cornflour and lemon juice. Whisk the egg whites with an electric whisk until you have a firm, smooth meringue. Fill a piping bag fitted with a plain nozzle with the meringue. Pipe the mixture onto a baking tray lined with baking paper.
Bake at 90°C for 2 hours 30 minutes, then leave in the switched-off oven for a further 30 minutes.
Using a kitchen grater, lightly sand the meringues so that they stand upright when turned over.
Melt a little white chocolate in the microwave and use a pastry brush to coat the surface, to protect it from moisture.
Using an electric whisk, whip the very cold single cream with the mascarpone. Use single cream with a minimum fat content of 30% (I often use Président’s Fleurette Extra cream). Gradually add the caster sugar. Fill a piping bag fitted with a plain nozzle with this chantilly cream.
Assembly
Pipe the chantilly cream onto the meringues.
Next, spoon the blueberry compote into the centre of the whipped cream.
Arrange the blueberries on top. I added some small mint leaves.