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1
Make the shortcrust pastry shell: rub the cold butter into the flour until the mixture resembles coarse sand. Add the egg yolk and cold water and bring the dough together without overworking it. Wrap and rest in the refrigerator until firm to the touch.
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2
Roll the pastry out and line a 23 cm loose-bottomed tart tin. Blind-bake at 190 °C until the base is fully cooked through and dry to the touch — this step is where the dish lives or dies: an unbaked pastry base in contact with the liquid syrup filling becomes irreparably soggy and no subsequent baking will correct it.
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3
Cut lattice strips from any remaining pastry and reserve on a tray in the refrigerator.
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4
Warm the golden syrup gently until it is fluid and coats a spoon in a thin, even sheet — cold syrup is too viscous to distribute uniformly through the breadcrumbs.
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5
Combine the warmed golden syrup, fresh white breadcrumbs, lemon zest, lemon juice, and double cream. Stir until each crumb is evenly coated. The mixture should be uniformly glossy with no dry pockets visible.
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6
Pour the filling into the blind-baked shell to a depth of 1.5–2 cm. Lay the lattice strips over the top — the open lattice allows steam to escape, preventing bubbling that would disturb the filling surface.
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7
Bake at 190 °C until the edges are just set and the centre retains a slight trembling wobble when the tin is moved. The filling will firm on cooling; a filling that is fully set in the oven will produce an over-hard, dry slice on the plate.
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8
Remove from the oven and allow to cool until warm — not hot. The correct chewy, fudge-like set develops as the tart cools. Serve in small, precise slices with cold clotted cream or cold vanilla custard poured generously at table.