The only place I ever heard this old-fashioned candy mentioned was on an episode of 1970's British TV drama When the Boat Comes In. But the Guardian published a recipe on Friday, and it was so easy I knew I had to try it:
100 g sugar
3 tablespoons golden syrup
1 teaspoon bicarbonate of soda
Step 1. Mix the sugar and golden syrup together in a saucepan. Step 2. Put over a low heat and stir until all sugar crystals have dissolved. Once they have dissolved, stop stirring and heat to 150 degrees C. Step 3. Add the bicarbonate of soda, take it off the heat, and stir like mad. Step 4. Pour the gooey mess onto a baking sheet lined with greaseproof paper or buttered tinfoil. Step 5. Leave to cool and break into pieces. Store in an airtight container. Or your mouth.
*** Editor's note: I took my eyes off the hot sugar for one instant, and it burned slightly, which gives my cinder toffee the unmistakeable taste of actual cinders. Please be mindful around hot candy!