Place the butter in a bowl, whisk in the sugar and salt using the whisk attachment on a hand mixer. Whisk in the egg and vanilla. Add the flour, mix quickly to form a soft pastry dough – do not knead. Flatten the pastry, cover and chill for approx. 30 mins.
Roll out one third of the pastry on the tin base, close the tin frame. Shape half of the remaining pastry into a roll and place around the edge of the pastry base, press with the fingertips and pull upwards approx. 4 cm. Prick the base firmly with a fork. On a lightly floured surface, roll out the remainder of the pastry to make a lid (approx. 4 mm thick) to match the size of the tin, place on a sheet of baking paper. Chill the base and lid for approx. 30 mins.
Boil the sugar and water in a wide pan without stirring. Reduce the heat and simmer, swirling the pan occasionally until a light brown caramel has formed, remove from the heat and allow to cool slightly. Pour in the cream, continue to simmer until the caramel has dissolved. Add the walnuts and salt, mix, simmer for approx. 2 mins., leave to cool. Spread the nut filling over the pastry base.
To cover the tart
Brush the edges of the pastry with water. Place the lid on top, press down firmly with a fork, prick.
Approx. 35 mins. in the lower half of an oven preheated to 180°C. Switch off the oven and leave the tart to continue baking for approx. 15 mins. Remove from the oven, allow to cool slightly, remove the tin frame. Slide the tart onto a cooling rack, leave to cool.