Mix the flour, sugar and salt in a bowl. Add the butter and rub together using your hands to form an even, crumbly mixture. Add the egg, mix quickly to create a soft dough – do not knead. Flatten the dough, cover and chill for approx. 30 mins. Roll the dough into a circle (approx. 28 cm in diameter) between two sheets of baking paper. Remove the upper sheet, place the dough and baking paper in the tart tin. Prick the base firmly with a fork, cover with the removed sheet of baking paper and weigh down with dried pulses.
To blind bake
Approx. 15 mins. in the lower half of an oven preheated to 200°C. Remove the baking paper and pulses, bake for a further 10 mins. Remove from the oven, allow to cool slightly, slide onto a cooling rack and leave to cool completely. Spread the cake icing over the tart base, chill for approx. 10 mins.
Mix the chestnut puree and milk until smooth, carefully fold in the whipped cream. Spread 3/4 of the custard over the tart base, smooth down. Transfer the remainder of the vermicelli custard to a piping bag with a serrated nozzle, chill both.
Place the nuts, sugar and water in a wide pan, bring to the boil without stirring. Reduce the heat and simmer for approx. 5 mins., stirring occasionally until the water has evaporated and the sugar is dry. Continue to stir with a wooden spoon until the sugar melts again and coats the walnuts, leave to cool. Pipe the remainder of the vermicelli custard on top of the tart, decorate with the nuts and meringue.