I melted 125g unsalted butter, 100g dark chocolate and 1 tablespoon of golden syrup in a saucepan over low heat, until the ingredients combined. When it combined, I removed it from the heat for 10 minutes and whisked in an egg. I then poured this chocolate mixture over the dry ingredients.
Though the recipe asked for a 17cm x 27cm slice tin, I found the ingredients didn't extend that far. Lucky the mixture was extremely accommodating and was easily moulded into shape. Here is the photo of the mixture moulded to half a standard slice tin.
I then topped the slice with 200g of dark chocolate and 50g of butter and popped the pan into the fridge for a couple of hours to set.