Step 1

In a stand mixer fitted with a paddle attachment, combine the butter, powdered and granulated sugars and vanilla bean seeds, and beat until light and fluffy. Add the egg yolks and vanilla extract and beat until the eggs are incorporated. Stir in the flour and salt just until combined.

Step 2

Divide the dough in half and form each half into a 10-by-1 ½-inch (25 by 4 cm) log. Wrap in parchment paper and refrigerate for at least 2 hours and up to 3 days. (The logs can also be frozen for up to 3 months.)

Step 3

Heat the oven to 350 degrees. Line two baking sheets with parchment paper.

Step 4

Working with one log at a time, unwrap and brush the outside with the egg white, reserving some for brushing each sablé. Sprinkle with half the turbinado sugar until completely coated. Using a sharp knife, cut the log into 32 thin slices, about ¼-inch (6 mm) thick, and transfer half to each prepared baking sheet. Brush the tops of the cookies with egg white and sprinkle with additional turbinado sugar and flaky salt.

Step 5

Bake, rotating the baking sheets top to bottom and front to back halfway through cooking, until the sablés are set and lightly golden around the edges, 12 to 15 minutes. Transfer the baking sheets to wire racks and let the cookies cool for 1 minute before transferring directly to the racks to cool completely. Repeat with the remaining dough log, egg white, turbinado sugar and flaky salt.