Combine the flour, salt, and cold, cubed butter in a mixing bowl. Rub the butter into the flour until sandy.

Stir in the sugar and add the whole egg. Bring the mixture together into a ball, wrap in plastic wrap, and refrigerate for 30 minutes.

Preheat oven to 180°C (350°F). Roll out the chilled dough on a floured surface and fit it into a buttered tart pan. Prick the bottom with a fork.

Precook the crust in the oven for 15 minutes until just starting to set.

In a saucepan, combine the milk and heavy cream with the split vanilla bean (scrape seeds). Heat gently until steaming.

In a bowl, whisk egg yolks with 100 g of sugar and cornstarch until smooth. Gradually pour in the warm milk mixture while whisking, then return it to the saucepan over low heat.

Stir until thickened, then add the lemon zest.

Pour the cream filling into the precooked crust. Bake for 25-30 minutes until golden brown with a gentle jiggle.

Allow to cool, then chill in the refrigerator for at least three hours before serving.