Make the dough by pulsing the flour, powdered sugar, and cold butter in a stand mixer with the paddle until the mixture resembles coarse crumbs. Add the egg yolk and mix just until the dough comes together. Wrap and chill for at least 1 hour.
On a lightly floured surface, roll the dough out to about 1/8 inch/3 mm thick and fit it into a 9-inch/23-cm tart pan, pressing the edges to form a sharp 90° corner. Freeze until firm, about 20–30 minutes.
Heat the oven to 340°F/170°C.
Prick the bottom of the tart shell with a fork, line with parchment and pie weights, and bake until pale golden, about 18–22 minutes.
Remove weights and bake 2–3 minutes more if needed. Let cool slightly.
Whisk together cream, milk, yolks, sugar, vanilla, cornstarch, and butter until smooth. Cook gently in a saucepan over low heat, stirring constantly, until it just thickens and coats the back of a spoon.
Lower the oven temperature to 300°F/150°C. Pour the filling into the baked shell and bake until set at the edges but still wobbly in the center, about 12–16 minutes.
Cool completely, then refrigerate until firm, at least 1–2 hours.
Before serving, sprinkle sugar evenly over the top and caramelize with a kitchen torch until golden and glassy.