Preheat the oven to 180C/160C Fan/Gas 5. Line an 8" square baking tray.
If making your own, make the pastry. Combine the butter and flour, either using your fingers to 'rub together' or by placing in a food processor and combining until the mixture resembles breadcrumbs.
Add the water until the mixture comes together to form a ball. Chill 10 mins in the fridge while the oven gets hot.
Roll the pastry on a lightly floured surface to about 1/2 inch thick. Place into the lined tin, pushing into all the corners. Cover this with a layer of tin foil or baking paper. Add baking beans or dried beans/dried rice to create a heavy layer to prevent the pastry from puffing up unevenly.
Bake for 15 minutes, then remove the foil/paper with the baking beans and bake a further 10 minutes. Remove and let cool. (Keep the oven on)
Make the cornflake layer while this cools: add the butter, sugar and golden syrup to a medium saucepan over a low/medium flame. Once everything has melted and combined, turn off the heat and add the cornflakes. Stir together.
Spread the jam over the pastry base, creating an even layer. Pour over the cornflake layer. Bake for 5-10 minutes until the cornflakes go crisp on top. Remove and let cool before slicing.