Preheat your oven to 180℃ (or 160℃ for fan ovens).
Line an 8x10-inch tin with the ready-rolled pastry and cut off any excess. Dock the dough by pricking it all over with a fork.
Cover the pastry with baking paper and pour on some baking beans (or uncooked rice/lentils) to help weigh it down. Blind bake for 15 minutes.
Remove the baking beans and baking paper and bake the pastry for a further 10-15 minutes until golden. You can cover the edges with tin foil to prevent bruning. Set aside to cool.
Put the butter, brown sugar and golden syrup into a saucepan. Place it over a low heat on the stove, stirring occasionally until melted.
Pour the melted mixture into a large bowl with the Cornflakes and mix well until all of the Cornflakes are coated.
Mix the strawberry jam to loosen it then spread it evenly over the shortcrust pastry. Top with the golden syrup Cornflake mixture.
Return the traybake to the oven for 5 minutes. Leave it to cool completely before cutting it into slices.