Put the sugars, syrup, salted caramel and butter in a heavy-bottomed pan with 3 tbsp water. Generously grease a shallow tin, about 20cm square, with butter.
Put the pan over a gentle heat and stir until the butter has melted and the sugars dissolved. Turn up the heat slightly and bring to the boil. Bubble until the mixture reaches 138C on a sugar thermometer and is a deep golden colour. This can take up to 10 minutes.
When the mixture reaches the temperature, take the pan off the heat and quickly stir in the bicarbonate of soda. The caramel will puff up.
Pour the bubbling mixture into the tin and leave to set for at least 1 hour.
When cold and firm, break into rough shards and store in an airtight jar.
The toffee will keep in an airtight container for up to 1 week.
Sugar thermometers make toffee and caramel recipes quick and easy. They’re not expensive and ensure a perfect result every time.
The tray size doesn’t need to be exact. Use one you might have for brownies or flapjacks.