Pre-heat your oven to 180°c (160°c for fan assisted ovens or Gas Mark 4) and generously grease a 11x7inch tray with some butter.
In a large bowl, mix the flour and sugar before adding the butter and rubbing together until it resembles breadcrumbs. Knead the mixture together to form a dough.
Using your fingers, press the dough into your pre-greased tin, trying to work it into as smooth a finish as possible. Prick holes all over the dough before baking in your pre-heated oven for 20 minutes.
Meanwhile, place the butter, sugar, condensed milk and golden syrup into a pan and gently heat until the sugar has disolved.
Bring the caramel mixture to the boil, stirring continuously. Reduce to a simmer, continuing to stir for 5 minutes, or until the caramel has thickened.
Pour the caramel over your baked base, spreading evenly across. Leave to cool.
Once completely cool, melt the chocolate over a bain marie or in short bursts in the microwave before pouring and spreading over the caramel to finish.
Leave to set in the fridge for at least 2 hours but ideally over night before cutting into individual portions.