Step 1: Preheat your oven to 180°C (350°F) / Fan 160°C / Gas Mark 4. Lightly grease a 23cm (9-inch) flan tin or pie dish.
Step 2: Roll out the shortcrust pastry (if using ready-made, simply unroll) and carefully line your prepared flan tin. Trim any excess pastry, leaving a slight overhang if desired, or crimp the edges. Prick the base all over with a fork.
Step 3: Spread the raspberry jam evenly over the base of the pastry. Set aside.
Step 4: In a saucepan, melt the butter, caster sugar, and golden syrup over a low heat, stirring until the sugar has dissolved and the mixture is smooth. Remove from the heat.
Step 5: Add the cornflakes to the saucepan with the butter mixture and gently stir until all the cornflakes are thoroughly coated.
Step 6: Spoon the coated cornflakes evenly over the jam layer in the pastry case. Gently press down slightly with the back of a spoon to create a compact topping.
Step 7: Bake in the preheated oven for 20-25 minutes, or until the pastry is golden brown and the cornflake topping is slightly crisp and golden. Keep an eye on it to prevent burning. If browning too quickly, you can loosely cover with foil.
Step 8: While the tart bakes, prepare the custard. In a medium saucepan, gently heat the milk until it's simmering (do not boil).
Step 9: In a separate bowl, whisk together the caster sugar, cornflour, egg yolk, and a splash of the warm milk until smooth. Slowly pour the remaining warm milk into the egg mixture, whisking constantly.
Step 10: Return the mixture to the saucepan and cook over a medium-low heat, stirring continuously with a whisk, until the custard thickens to your desired consistency. Remove from heat and stir in the vanilla extract.
Step 11: Allow the Cornflake Tart to cool slightly in the tin before slicing. Serve warm with plenty of the freshly made hot custard.