Line a shallow baking tray with non-stick baking parchment.
Measure all the ingredients into a deep, heavy-based pan and place over a gentle heat. 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 140C on a sugar thermometer then pour into the lined tray.
Wait until the toffee is cool enough to handle then cut into even pieces with scissors. Roll each piece of toffee into a ball or fat log shape and wrap in waxed paper. Store in an airtight jar.
The toffee will keep in an airtight container for up to 1 month.
Sugar thermometers make toffee and caramel recipes quick and easy. They’re not expensive and ensure a perfect result every time.