Prepare Your Tin: Line a 22x33cm (9x13 inch) baking tin with a single sheet of baking parchment, making sure it goes up the sides. This will make it easy to lift the toffee out later.

Toast the Nuts: Spread the chopped pecans on a small baking tray and toast in a preheated oven at 180°C (160°C fan) for 5-7 minutes, or until fragrant and slightly darkened. Set aside.

Melt and Dissolve: In a medium, heavy-bottomed saucepan, combine the butter, caster sugar, water, golden syrup, and salt. Place over a medium-low heat and stir gently with a wooden spoon until the butter has melted and the sugar has completely dissolved. You shouldn't feel any grainy texture when you rub a little of the liquid between your fingers.

Boil to Temperature: Once the sugar is dissolved, stop stirring. Attach a sugar thermometer to the side of the pan. Increase the heat to medium-high and bring the mixture to a steady boil.

Watch and Wait: Continue to boil the mixture, without stirring, until the thermometer reaches the "hard crack" stage, which is 149°C (300°F). The mixture will be a beautiful deep amber colour. What works best for me is to just gently swirl the pan occasionally to ensure even cooking. This stage can take 10-15 minutes.

Final Additions: As soon as it hits temperature, immediately remove the pan from the heat. Working quickly and carefully, whisk in the vanilla extract and bicarbonate of soda. The mixture will foam up vigorously – this is normal!

Pour and Set: Instantly pour the hot toffee mixture into your prepared baking tin. Use an offset spatula or the back of a spoon to spread it into an even layer. Be quick as it will start to set almost immediately.

Melt the Chocolate: Let the toffee sit for 2-3 minutes to cool just slightly, then evenly scatter the chopped dark chocolate over the surface. The residual heat from the toffee will melt the chocolate within a few minutes.

Spread and Top: Once the chocolate is soft and glossy, use an offset spatula to spread it into a smooth, even layer covering the entire surface of the toffee. Immediately sprinkle over your toasted pecans and the flaky sea salt.

Cool and Break: Allow the toffee to cool and harden completely at room temperature for at least 2 hours, or speed up the process by placing it in the fridge for about 30-40 minutes. Once solid, lift the parchment paper out of the tin and break the toffee into irregular shards.