Preheat the oven to 180°C (160°C Fan / Gas Mark 4) and lightly grease a 20cm (8-inch) round fluted tart tin if needed.
Unroll the shortcrust pastry and press it into the prepared tart tin, trimming any excess and making a neat edge. Prick the base all over with a fork.
Blind bake the pastry base for 10–12 minutes, or until lightly golden. Remove from the oven and let cool slightly.
Spread raspberry jam evenly over the base of the warm pastry.
In a medium saucepan over low heat, melt the unsalted butter, golden syrup and caster sugar, stirring until combined and sugar is dissolved.
Remove the pan from the heat, add the cornflakes and gently fold until all flakes are evenly coated.
Spoon the coated cornflakes over the jam layer, pressing down lightly to compact the topping.
Bake for 10–15 minutes, or until the topping is golden brown and caramelised, watching carefully to avoid burning.
Allow the tart to cool in the tin for 15–20 minutes to set the topping before slicing.
Serve warm with hot custard, vanilla ice cream or on its own alongside a cup of tea.