Grease a 20cm square tin with the butter. Mix the caster sugar and syrup in a deep saucepan and stir over a gentle heat until the sugar has melted. Try not to let the mixture bubble until the sugar grains have disappeared. Once completely melted, turn up the heat a little and simmer until you have an amber coloured caramel (this won’t take long), then as quickly as you can turn off the heat, tip in the bicarbonate and beat in with a wooden spoon until it has all disappeared and the mixture is foaming. Scrape into the tin immediately but be careful as the mixture will be very hot.
The mixture will continue bubbling in the tin, simply leave it and in about 1 hr- 1 hr 30mins the honeycomb will be hard and ready to crumble or snap into chunks.