Broyé du Poitou (French Shortbread Cookie Cake)

Step 1: Trace a 10-inch circle on a piece of parchment to make a stencil; a large dinner plate is a helpful guide. Turn the parchment over and place it on a baking sheet, so you can see the circle through the paper.

Step 2: In a large bowl, use a flexible spatula to beat the butter and sugar together until well mixed and lighter in color, about 1 minute. Alternatively, combine them in the bowl of a stand mixer fitted with the flat beater and mix on medium speed for about 1 minute.

Step 3: Add the vanilla, salt, and whole egg; beat vigorously, or on medium speed with a mixer, until smooth, about 1 minute. Add the flour and mix just until incorporated, scraping the sides of the bowl. If using a mixer, use low speed.

Step 4: Place the dough in the center of the prepared parchment. Using your hands, pat it into an even 10-inch round about 1/4 to 1/3 inch thick, using the traced circle as a guide.

Step 5: To make a scalloped border, hold a wooden spoon or flexible spatula vertically, with its handle touching the edge of the cookie cake and the parchment below. Press the handle toward the center about 1/2 to 1 inch to make an indent. Repeat around the outer edge until the border is complete.

Step 6: Place the baking sheet with the cookie cake in the refrigerator to chill for 20 minutes. Meanwhile, preheat the oven to 350°F with a rack in the center.

Step 7: Remove the cookie cake from the refrigerator and brush the egg wash evenly over the surface. Use the tines of a fork to score a diagonal or crisscross pattern on top.

Step 8: Bake for 25 to 30 minutes, or until golden brown. Remove from the oven and let it cool slightly on the pan, about 10 minutes. Use the parchment to slide the cookie cake onto a wire rack. Cool completely before breaking into pieces to serve.

Step 9: Store leftovers in an airtight container at room temperature for 3 days, or freeze for longer storage.