Golden and rustic, Broyé du Poitou (aka broyé poitevin) may look like a buttery cake at first glance, but break into it and you’ll find something closer to a giant shortbread cookie. Native to western France’s Poitou region, this traditional pastry skips the leavening to create a pleasantly dense-yet-soft interior with a crisp, crackly surface. The cookie-cake’s elegant scalloped edges and decorative top belie its simplicity: It comes together in a single bowl with just six pantry staples. Serve it broken into shards (traditionally, the big cookie is punched in the center or broken up by hand into irregular pieces — never cut!) and enjoy it with coffee or tea — ideally, among friends.
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Create a stencil for the cookie cake by tracing a 10" circle on a piece of parchment (a large dinner plate is a helpful guide). Turn the piece of parchment over and place it on a baking sheet; you should be able to see the circle through the parchment.
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 the butter and sugar in the bowl of a stand mixer fitted with the flat beater, and mix on medium speed, about 1 minute.)
Add the vanilla, salt, and whole egg; beat vigorously (or on medium speed) until smooth, about 1 minute. Add the flour and mix (on low speed, if using a mixer), scraping the sides of the bowl, just until incorporated.
Transfer the dough to the prepared parchment, placing it in the center of the circle. Using your hands, pat the dough into an even 10" round about 1/4" to 1/3" thick, using the traced circle as a guide.
To create a scalloped border, hold a wooden spoon or flexible spatula vertically so that the handle touches the side of the cookie cake and the parchment below. Press the handle in toward the center about 1/2" to 1" to create an indent. Repeat the process, working your way around the outer edge until the border is complete.
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.
Remove the cookie cake from the refrigerator and brush the egg wash evenly over the surface (this helps gives the cookie cake a shiny, golden brown top). Use the tines of a fork to score a diagonal or crisscross pattern onto the surface.
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), then use the parchment to slide the cookie cake onto a wire rack. Cool completely before breaking into pieces to enjoy.
Store leftover cookie cake in an airtight container at room temperature for 3 days; freeze for longer storage.
