Line and butter a 20 x 20 centimetre tray. Put the sugar and syrup into a large saucepan over a low heat and stir until the sugar has dissolved, then let it heat to 150C. Make sure you use a saucepan large enough to hold the syrup when it expands with the bicarbonate of soda. Once the syrup reaches temperature, quickly take it off the heat and add the bicarbonate of soda. Stir gently with a whisk until the bubbles are evenly dispersed then pour the mixture into the prepared tray and leave to set for 2 hours.
Turn the honeycomb out onto a chopping board. Use a serrated knife to carefully cut 2 centimetre wide strips from the honeycomb. Take your time sawing through the honeycomb as it’s prone to shattering and you will probably end up with a few broken pieces regardless.
Once you’ve sliced the bar, line a tray with greaseproof paper and submerge each piece of honeycomb in the melted chocolate. Use two forks to lift it out and drain any excess chocolate. Transfer to the lined tray, repeat with the rest and leave to cool for around 1 hour until set.