Preheat the oven to 180C/Fan 160C/Gas 4 and place the hazelnuts on a large baking tray in a single layer, roast for 10 minutes. They should be finished a few minutes before the fudge.
While the hazelnuts are roasting, line a 20cm baking tin with baking paper and place a glass of cold water in the fridge, this will be used to test the fudge temperature later. Also lay a lint free tea towel out on a counter as this will also be used later.
In a large, non-stick saucepan combine the condensed milk, butter, and sugar. Over medium/low heat bring this mixture to a boil, stirring constantly. Make sure to scrape the bottom of the pot to avoid the mixture catching.
Continuously stir the fudge for 10 minutes, adjusting the temperature as needed to avoid burning. Take care as the mixture will be extremely hot. During this time quickly remove the nuts from the oven and place on the tea towel, wrap the tea towel around them and leave them to steam for a few minutes.
After 10 minutes of stirring, test the fudge by dropping a small piece into the cold glass of water. The piece should drop to the bottom of the glass and form a ball or tear shape. Alternatively, the temperate should have reached 113C (soft ball stage). Set aside the fudge to cool for a few minutes.
At this point the nuts will be ready to husk. Rub the nuts inside the tea towel until their skins fall off (don’t worry about all of them coming off, just the majority).
Add the vanilla extract to the fudge and vigorously stir until the mixture firms up a little. This will take around 2-3 minutes. While you do this, microwave the hazelnut/chocolate spread for 10-15 seconds, just until it melts. (If you don’t have a microwave, melt in a heat-proof bowl over a small pan of water).
Pour the fudge into the lined baking tin and using the back of a metal spoon, push the fudge into an even layer. Drizzle over the hazelnut/chocolate spread and evenly place the hazelnuts on top, push them into the fudge a little so they don’t move. Finish with a generous pinch of sea salt.
Leave to set for 3 hours at room temperature, or 1 hour in the fridge before slicing into squares.