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Line a 20 x 20cm cake tin with baking parchment. Tip the sugar, cream, butter and glucose syrup into a medium to large saucepan. Heat to dissolve the sugar and melt the butter, stirring now and again.
Once dissolved, put a sugar thermometer in the pan, making sure the end is completely covered by the syrup – if not, transfer the mixture to a smaller pan (with enough space for the syrup to bubble up). Increase the heat and bring the syrup to a steady boil. Keep bubbling, stirring occasionally to stop the sugar from catching, until the mixture reaches 116C – this is known as the soft ball stage.
Remove the pan from the heat and leave to sit, undisturbed, for 5 mins, until the temperature drops to 110C. Stir in the vanilla and a good pinch of salt.
Keep the sugar thermometer in the pan and begin beating the mixture with a wooden spoon, quite vigorously, until the temperature cools to about 60C. By this time the fudge will be really thick and will have lost it glossy shine. Remove the thermometer and continue beating for a few mins more. This process is very important when making fudge, as it creates small sugar crystals, which give the fudge its lovely smooth and creamy texture (see 'fudge know-how', below).
Before it sets completely, quickly pour the fudge into your prepared tin and smooth over the surface. Leave to cool at room temperature overnight – don’t put the fudge in the fridge as it will become sticky and won’t set properly. Cut into bite-sized pieces and pop in a box to give as a present. Will keep, in a sealed container, for up to 2 months.