Rub the butter into the flour, icing sugar, vanilla and salt until it resembles breadcrumbs. Stir in the egg yolk and a little water until it comes together. Wrap and chill for 30 minutes.
Roll out the pastry and line a 23cm tart tin. Chill again for 15 minutes.
Line with baking parchment and fill with baking beans. Bake at 180°C (fan 160°C) for 20 minutes, remove the beans, and bake another 10 minutes until golden. Trim the edges.
Heat the double cream and vanilla in a saucepan until steaming (not boiling).
In a bowl, whisk together the egg yolks, salt and sugar until pale.
Slowly pour the hot cream over, whisking constantly, then strain through a fine sieve.
Pour the custard into the tart case, grate nutmeg generously over the top, and bake at 130°C (fan 110°C) for 35–40 minutes, or until just set with a slight wobble in the centre.
Allow to cool completely before slicing
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