- Butter a 23 x 13-cm (9 x 5-inch) loaf pan and line it with parchment paper.
- In a heavy saucepan, combine all the ingredients, except for the vanilla. Bring to a boil over medium heat, stirring until all the sugars have dissolved. Attach a candy thermometer in the centre of the pan and simmer over medium heat, stirring gently until the thermometer reads 132 °C (269 °F). Remove from the heat and stir in the vanilla. Pour into the pan and let cool for about 30 minutes. Unmould.
- With a knife dipped in hot water, cut the caramel into cubes. Once the caramel has completely cooled, it will become brittle. Keep the candy in an airtight container separated with sheets of parchment paper.
It is preferable to stir gently, otherwise, the syrup might stick and burn at the bottom of the pan. Use a clean spoon each time.
Do not wait over an hour otherwise the caramel will become too hard and impossible to cut into nice pieces.
You can sprinkle the caramel with crushed toasted walnuts while it is still warm.