Place the butter in a bowl, whisk in the sugar and salt using the whisk attachment on a hand mixer. Whisk in the water and vanilla. Add the flour, mix quickly to form a soft dough – do not knead. Flatten the dough, cover and chill for approx. 30 mins.
On a lightly floured surface, roll the dough into a circle approx. 2 mm thick, place in the prepared tin, prick the base firmly with a fork, chill for approx. 15 mins. Cover the dough with baking paper, weigh down with dried pulses.
To blind bake
Approx. 10 mins. in the lower half of an oven preheated to 200°C. Remove the baking paper and pulses, bake for a further 15 mins. Remove from the oven, reduce the heat to 160°C. Allow the tart base to cool slightly on a cooling rack.
Heat the cream and milk with the grapefruit zest and ginger, leave to infuse for approx. 30 mins. Bring the liquid to the boil, allow to cool slightly. In a bowl, mix the sugar with all the other ingredients up to and including the salt. Slowly pour in the milky mixture while stirring, pour through a sieve. Pour the filling into the tart base, place the tart on a tray.
Approx. 30 mins. in the centre of an oven preheated to 160°C. Remove from the oven, place on a cooling rack, leave to cool in the tin. Carefully remove the tart from the tin. Sprinkle the sugar evenly over the tart just before serving, caramelize with the flame on a blowtorch.