Pulse the flour, butter and icing sugar in a food processor until the mixture resembles coarse breadcrumbs. Add the yolk and water, pulse just until it forms a ball. Wrap in cling film and refrigerate 1 hour.

Roll the pastry to 3mm thick on a lightly floured bench. Line a 24cm fluted tart tin, trim the edges. Prick the base lightly. Refrigerate 20 minutes.

Preheat oven to 180°C fan-forced. Line the pastry with baking paper and fill to the brim with rice, dried beans or ceramic baking weights. Bake 18 minutes.

Lift out the paper and weights. Brush the base and sides with the beaten egg yolk. Return to the oven for 8 minutes until pale gold. Cool completely on a rack.

Combine the lemon zest, juice, sugar, eggs and extra yolk in a heatproof bowl set over a saucepan of simmering water (the bowl shouldn’t touch the water).

Whisk constantly for 8 to 10 minutes until the curd thickens and coats the back of a spoon. Remove from the heat.

Whisk in the cold butter cubes one at a time until smooth and glossy. Strain through a fine sieve into a clean bowl.

Pour the warm curd into the cooled tart shell. Smooth the top with the back of a spoon. Refrigerate at least 2 hours, ideally overnight.

Just before serving, sprinkle the caster sugar in a thin even layer across the surface of the chilled tart.

Hold a butane torch 10cm above the surface. Move steadily across the tart until the sugar melts, bubbles and turns deep amber. Stop just before mahogany.

Let the sugar cool for 90 seconds until brittle. Slice with a hot knife wiped between cuts. Serve immediately.