Preheat the oven to 150C (gas mark 2). Line a 23cm square tin with baking tray liners. I like to cross them over one another, to avoid leakage.
Add the sugar and pulse again, until a dough has formed. Then pat this dough into the tin.
Bake in the oven for 35 minutes, until lightly brown. Leave it to cool.
Combine the condensed milk, golden syrup and margarine in a saucepan on a low-medium heat. Bring this gradually to the boil, ensuring that the ingredients melt together fully. Then stir every 2-3 minutes, to make sure that it doesn't catch or burn. Once the caramel has thickened and is golden brown then remove it from the heat and leave it to cool slightly.
Pour the caramel mixture on to your baked shortbread. Place this into the fridge for 3-4 hours until it has cooled completely/set.
For the topping melt the chocolate in a bowl over a saucepan of boiling water.
Pour the mixture on the top of the set caramel and turn the tray slowly in four directions to spread the mixture out properly.
Place this back in the fridge for 20-30 minutes, to set. Then I carved the slices and wiped the knife in between each slice to keep it as clean as possible.
I placed this back in the fridge for 1-2 hours to make the perfect slice that was easy to serve.