In a mixing bowl, combine all-purpose flour, granulated sugar, and a pinch of salt. Cut in cold butter until the mixture resembles coarse sand. Add an egg yolk and cold water, mixing until a dough forms. Press it into a tart pan and chill in the fridge for 20-30 minutes.

Preheat your oven to 350°F (175°C). Remove the chilled crust from the refrigerator and prick it all over with a fork. Bake for 12-15 minutes until the edges are golden brown. Allow it to cool slightly on a wire rack.

In a saucepan over medium heat, combine granulated sugar, cornstarch, lemon zest, and egg yolks, whisking until smooth. Gradually mix in the milk until there are no lumps. Stir continuously until it thickens and bubbles, about 6-8 minutes. Remove from heat and stir in fresh lemon juice, unsalted butter, and vanilla extract.

Carefully pour the custard into the cooled crust, smoothing the top with a spatula. Bake in the oven for 25-30 minutes, or until the edges are set with a slight jiggle in the center. Let it cool completely on a wire rack.

Once cooled, slice the cake into wedges and serve with whipped cream or fresh berries.