First make the pastry. Rub the butter into the flour and sugar with your fingertips, then gradually add the egg until just coming together (be careful not to overwork the pastry or it will become tough).
Form your dough into a rough ball and cut in half. Cover both halves with plastic wrap/clingfilm and pop one in the freezer, for use at a later date, and chill one in the fridge for 30 minutes.
Preheat the oven to 190°C/ Fan 170°C/Gas 5. Take your pastry out of the fridge and roll out on a lightly floured surface so that it it’s big enough to fit a 18–20cm tart tin with some overhang.
Transfer the pastry to your butter-greased tart tin and use your fingers to carefully push the pastry into the sides – don’t worry about any pastry overhang as we’ll trim that later to make it look nice.
Line the tart case with baking paper and fill with baking beans or uncooked rice, then blind-bake for 15 minutes.
While the pastry is baking, make the filling. In a large saucepan over a low heat, gently warm the golden syrup and maple syrup until loose and runny – take care not to let it boil or the sugars will burn.
Take off the heat and add the breadcrumbs, lemon zest and juice and the butter. Stir until the breadcrumbs have absorbed the syrup.
Remove the paper and beans from the tart case and bake for a further 5 minutes to dry out the base of the tart case. Take the tart case out of the oven, then turn the temperature down to 160°C/Fan 170°C/Gas 5.
With a sharp knife, carefully trim the overhanging edges of the tart case. Pour the filling into the tart case and bake for 25–30 minutes, until a deep, golden colour. Serve with cultured or clotted cream to give your arteries an extra challenge.