Grease a 20cm square, deep cake tin and line with baking parchment. Grease the parchment.
Put the sugar and golden syrup in a non stick saucepan.
Heat gently until the sugar crystals dissolve. Don't stir but give the pan a little swirl every now and again.
When the sugar has dissolved turn the heat up until you have a bubbling liquid and heat until it reaches 140c/300F on a sugar thermometer or until your caramel turns golden. This takes about 3 minutes.
Add the bicarbonate of soda and whisk quickly to incorporate, the toffee will bubble up volcanically, transfer it straight into your prepared tin and leave to cool completely.
Break into chunks.
Store in an airtight tin for up to 3 days.