In a mixing bowl, combine all-purpose flour, granulated sugar, and a pinch of salt. Using a pastry cutter or your fingers, incorporate cold butter until the mixture resembles coarse sand. Add the egg yolk and drizzle in ice-cold water, mixing until a dough forms. Firmly press the dough into a tart or square cake pan, then chill for 20–30 minutes to set.

Preheat your oven to 350°F (175°C). Once the crust is chilled, use a fork to prick the surface, preventing bubbling during baking. Place the crust in the preheated oven and bake for 12–15 minutes or until it turns a lovely golden brown. Once baked, remove from the oven and let it cool slightly before adding the custard.

In a saucepan, whisk together sugar, cornstarch, lemon zest, and egg yolks until smooth and well combined. Gradually whisk in whole milk over medium heat, stirring constantly until the mixture thickens and coats the back of a spoon, about 5-7 minutes. Remove from heat and stir in freshly squeezed lemon juice, butter, and vanilla extract for a glossy finish.

Carefully pour the vibrant lemon custard into the prepared crust, smoothing the top with a spatula. Return the pan to the oven and bake for 25–30 minutes, or until the edges are set and the center jiggles slightly when shaken. Keep an eye on it to avoid overbaking, as the custard will firm up as it cools.

After baking, let the Lemon Custard Cake cool completely in the pan at room temperature. For the best flavor and texture, refrigerate the cake for at least 2 hours before slicing. Once chilled, dust with powdered sugar, and garnish with lemon peels or fresh berries before serving.