Grease a baking tray and line with baking paper.
Put the sugar and 125ml boiling water in a pan. Simmer over a low heat until the sugar has dissolved, gently swirling the pan.
Add the cream of tartar, treacle and syrup, and swirl the pan to mix. Bring to the boil and simmer for 25-30 mins until the bubbles are small and the mixture is thick. Mix in the black colouring, then pour onto the tray and leave to cool.
Smash the toffee into bite-sized pieces and enjoy!