Grease and line a 20cm square tin with baking paper.
Put the sugar and water in a large, heavy-based saucepan and heat gently, stirring, until the sugar has completely dissolved.
Add the black treacle, golden syrup, butter and cream of tartar and stir until the butter has melted.
Bring to the boil and boil without stirring until the mixture reaches 150°C on a sugar thermometer, the hard crack stage. This takes 10 to 15 minutes.
Immediately pour the toffee into the prepared tin and leave to set at room temperature for about 1 hour.
Lift out the toffee and break it into shards with a toffee hammer or rolling pin. Store in an airtight container.