Step 1: Prepare the tart crust 

1. Combine dry ingredients: In a big bowl, combine the all-purpose flour and sugar.
2. Cut in the butter: Add the cold, cubed butter and rub it into the flour mixture using your fingertips until it resembles coarse breadcrumbs.
3. Form the dough. Stir in the egg yolk and 1 to 2 tablespoons of cold water, until the dough just comes together. Avoid overworking it.
4. Chill the dough. Form the dough into a disk, wrap it in plastic and refrigerate for at least 30 minutes.
5. Roll and shape. On a floured surface, roll out the chilled dough into a 9-inch tart pan. Trim the edges and prick the bottom with a fork. Chill the crust in the refrigerator for 15 minutes.

Step 2: Pre-Bake the Crust

1. Preheat the oven to 375°F (190°C) 
2. Fit chilled tart crust with parchment paper and fill it with pie weights or dried beans.
3. Bake for 15 minutes, then take out weights and parchment paper. Continue baking for an additional 5-7 minutes, or until lightly golden. Set aside.

Step 3: Prepare the Vanilla Custard Filling

1. Heat the milk and vanilla: Split the vanilla bean lengthwise and scrape the seeds. In a saucepan over medium heat, heat the milk, vanilla seeds, and bean pod until just simmering, then remove from the heat. Let it sit for 5 minutes. (If using vanilla extract, you add it later.)
2. Whisk the egg mixture: In another bowl, whisk the egg yolks with the sugar and cornstarch until smooth. Slowly pour in the warm milk, whisking constantly to temper the eggs.
3. Cook the custard: Return the mixture to the saucepan and cook over medium heat, stirring constantly, until it thickens (about 3-5 minutes). Discard the vanilla bean pod, then whisk in the butter. (If using vanilla extract, add it now.)

Step 4: Assemble and Bake

1. Add the custard to the par-baked tart crust, using a spatula to smooth the top.
2. Bake the filled tart in a 325-degree-F (160-degree-C) oven for 15 minutes, or until the custard is set but still a little jiggly in the middle.
3. Let the tart cool to room temperature, then refrigerate until fully set, at least 2 hours.