Slowly heat the sugar and golden syrup in a small pan, stirring often. Let the sugar dissolve fully, then turn the heat up slightly until it begins to boil and turns a rich caramel/golden brown colour. Turn off the heat, add the bicarb and immediately begin stirring the mixture quickly, to make sure there are no pockets of bicarb hidden in the mix. It will begin to foam and expand. As soon as it’s fully combined, pour the mixture into the lined tin. It will continue to expand and will bubble a bit more.