Line 2 baking sheets with parchment paper. In a medium bowl, whisk together the flour and cornstarch; set aside.
Make vanilla sugar: Combine the granulated sugar with the seeds from the vanilla bean. Use your fingertips to rub the seeds into the sugar until evenly speckled and fragrant.
In a stand mixer fitted with the paddle (or with a hand mixer), beat the butter and vanilla sugar on medium speed until light and creamy, 2 to 3 minutes. Beat in the egg yolk, vanilla extract, and fine sea salt until smooth.
Add the flour mixture and mix on low just until the dough comes together in soft clumps with no dry spots. Finish by pressing the dough together with clean hands.
Divide the dough in half. Shape each portion into a log about 8 inches long and 1 1/4 inches thick. Wrap tightly in parchment or plastic wrap and chill until very firm, about 1 1/2 hours.
Heat the oven to 350°F. Brush each log lightly with milk or cream, then roll in turbinado sugar to coat the sides. Using a sharp knife, slice into 3/8-inch rounds and arrange on the prepared sheets, spacing about 2 inches apart. Sprinkle with a pinch of flaky sea salt if using.
Bake 12 to 14 minutes, rotating the pans halfway through, until the edges look set and the bottoms are just turning pale golden. The tops should remain light in color.
Cool on the baking sheets for 5 minutes, then transfer to a wire rack to cool completely. Store airtight at room temperature for up to 1 week.