Line with parchment and grease an 8 x 8 inch pan. Set aside.
Put the sugar and syrup into a deep saucepan and stir together to mix. Once the pan's on the heat, stop stirring!
Place the pan on the heat and let the mixture first melt and then turn to goo and then a bubbling mass the color of maple syrup - this will take 3 minutes or so.
Off the heat, whisk in the baking soda and watch the syrup turn into a whooshing cloud of aerated pale gold. Turn this immediately onto a piece of baking parchment or greased foil. I found to spread it, tilt the pan rather than spreading with a spatula.
Once the sugar has solidified, spread with melted chocolate. Let set until cool.