Prepare your tin. Line a 20cm square baking tin with baking parchment, leaving an overhang on two opposite sides. This will act as handles to lift the fudge out later.

Start the base. Place the caster sugar, cubed butter, evaporated milk, and golden syrup into a large, heavy-bottomed saucepan. Set it over a low heat. Smell: A sweet, buttery aroma will begin to fill your kitchen. Look: The butter will slowly melt into a pale, sandy liquid.

Dissolve the sugar. Stir gently with a wooden spoon until the sugar has completely dissolved. You should not hear any gritty sound when you stir against the bottom of the pan. This takes about 5 minutes of patient, low heat.

Bring to the boil. Increase the heat to medium and bring the mixture to a gentle, rolling boil. Clip a sugar thermometer to the side of the pan. Sound: The quiet simmer will become a louder, more active bubbling. Look: The liquid will become thick and frothy.

Cook to the soft-ball stage. Continue to boil, without stirring, until the thermometer reads 115°C (soft-ball stage). Look: The bubbles will become smaller and the mixture will darken to a light caramel colour. This takes about 10–12 minutes. Do not walk away; the temperature can rise quickly.

Cool the mixture. Remove the pan from the heat immediately. Add the sea salt and vanilla extract, but do not stir. Let the mixture sit untouched for exactly 5 minutes. Look: The surface will become still and glossy.

Add the chocolate and beat. Tip the chopped dark and milk chocolate into the pan. Begin to stir gently in one direction. Look: The chocolate will melt into dark ribbons. Texture: As you stir, the mixture will change from a thin liquid to a thick, matte, and grainy paste. This is the fudge "seizing" — it is ready when it holds its shape on the spoon and looks dull, not shiny.

Fold in extras and set. Quickly fold through the toasted hazelnuts. If using white chocolate chips, dot them on top and swirl gently with a knife. Pour the thick fudge into the prepared tin and spread it into an even layer. Sound: You will hear a soft, squelching sound as you press it into the corners. Leave to set at room temperature for at least 2 hours, or overnight for a cleaner cut.