Line a 27 × 40 × 2.5 cm(11 × 16 × 1 inch) baking tray with a double thickness of baking paper. Scatter the nuts in and flatten them with the back of a wooden spoon to make a compact even layer.
Put the butter and vanilla in a heavy-based saucepan and add a pinch of salt and 2 tablespoons of water. Heat until the butter melts and just starts to bubble, then add the sugar. Cook over low heat, stirring almost continuously to prevent it sticking to the bottom, until you have a thick rich toffee-coloured mass that comes away from the side of the pan when stirred — if you have a thermometer it will be at just below 150°C (300°F). It could take up to 20 minutes to get there, depending on your pan and the heat. If the toffee looks like separating at any point, take it off the heat and vigorously beat in 2 tablespoons of hot water to bring it together again, standing back as it may splash up.
Carefully pour the toffee over the nuts, pouring around the sides first and working your way in to cover all the nuts. Scatter the chocolate as evenly as possible over the hot toffee, then leave it for 5 minutes to soften before spreading it evenly with a spatula. Leave to cool and set completely — in the fridge if it’s very hot. Use the baking paper to lift it out of the tin. Break up into pieces and keep in a biscuit tin, or in the fridge if the weather’s hot.