Line a 15 x 20cm/6 x 8in baking tray with non-stick baking parchment.
Measure all the ingredients into a deep, heavy-based saucepan. Place over a medium heat and stir occasionally until the butter is melted and the sugar has dissolved.
When the mixture is smooth and well-combined, increase the heat and bring the mixture to a fast boil. When the mixture reaches 140C/285F, pour carefully into the lined tray.
If you want even pieces, wait until the toffee is cool enough to handle, but elastic enough so that a finger pressed in the top leaves a slight indent (about 15-20 minutes depending on the room temperature). Partially cut the toffee into even pieces with an oiled knife. When it has cooled completely, break along these lines. Alternatively let the toffee cool completely, then break into shards with a toffee hammer or rolling pin.
Store in an airtight container, with individual layers of toffee separated by baking parchment. If left exposed to humidity, the toffee will soften and stick together.