Add golden syrup to a large pan and heat over a medium heat until thin and runny. Add the sugar and heat gently (medium/low heat). Avoid stirring the mixture but you can shake the pan on occasion to ensure you don’t get pockets of sugar which caramelize too quickly. Once the sugar has melted, use a thermometer and bring the temperature up to 149C. Immediately take the pan off the heat and sieve the bicarbonate soda into the mixture and quickly whisk. The mixture will puff up and double in size. Tip carefully onto a large silicone baking sheet (placed on a cooling mesh). Once cool, break into shards. Store in an airtight container - it will keep for a month.
WARNING: Mixture becomes very hot!