Mix 1 ½ cups all-purpose flour, ½ cup granulated sugar, and a pinch of salt in a medium bowl. Cut in ½ cup cold butter until mixture resembles coarse sand. Add 1 egg yolk and 2-3 tablespoons cold water to form dough. Shape into a disc, wrap in plastic, and chill for 20-30 minutes.

Preheat your oven to 350°F (175°C). Roll out the chilled dough in a floured surface and fit into a 9-inch tart pan. Prick the base with a fork, bake for 12-15 minutes until golden, and let cool slightly.

In a saucepan, whisk together 1 cup sugar, ¼ cup cornstarch, and zest of 2 lemons. Gradually add 2 cups whole milk, whisking continuously. Heat over medium, stirring until thickened and bubbling (about 5-7 minutes).

Remove the saucepan from heat, stir in juice of 2 lemons, 2 tablespoons unsalted butter, and 1 teaspoon vanilla extract until smooth.

Pour the custard over the cooled crust, spreading evenly. Bake for 25-30 minutes until edges are set and center jiggles slightly.

Let the cake cool completely at room temperature (about 1-2 hours). Dust with powdered sugar and garnish with lemon slices or berries if desired. Slice and serve.