Preheat oven to 350°F (175°C). In a food processor, pulse the flour, sugar, and salt. Add the cold, cubed butter and pulse until the mixture resembles coarse crumbs. Add the egg yolk and pulse to combine. With the processor running, drizzle in ice water, 1 tablespoon at a time, just until the dough begins to clump. Turn dough out, shape into a disc, wrap in plastic, and refrigerate for 15 minutes. Roll out the dough on a floured surface to fit a 9-inch tart pan or springform pan. Press into the pan, trim edges, and prick the bottom with a fork. Line with parchment and pie weights or dried beans. Blind bake for 15 minutes. Remove weights and parchment and bake for another 5 minutes until lightly golden. Set aside to cool slightly.

In a large bowl, whisk the eggs and sugar together until smooth and slightly pale. Whisk in the flour until no lumps remain. Gradually whisk in the lemon juice and zest. Slowly whisk in the milk, followed by the melted butter, vanilla extract, and salt until the mixture is completely homogenous.

Place the par-baked crust on a baking sheet (to catch any potential drips). Pour the lemon custard filling into the warm crust. Carefully transfer to the oven. Bake for 35-40 minutes, or until the custard is set around the edges but still has a slight, gentle wobble in the very center. The top should be a light, golden yellow.

Remove the cake from the oven and let it cool completely on a wire rack. The custard will continue to set as it cools. Once at room temperature, you can refrigerate it for at least 2 hours for a firmer slice. Before serving, dust the top lightly with powdered sugar if desired.