For the Cookies:
2½ cups all-purpose flour
½ cup unsweetened cocoa powder
1 stick, plus 5 tablespoons, unsalted butter, softened
⅔ cup granulated sugar
¾ teaspoon fine sea salt
1 large egg white
1 teaspoon vanilla extract
For the Filling:
6 tablespoons unsalted butter, softened
1¼ cups confectioners’ sugar
2 teaspoons vanilla extract
¼ teaspoon fine sea salt
1. Make the cookies: Preheat the oven to 350° and line 2 baking sheets with parchment paper. In a medium bowl, whisk together the flour and cocoa powder until incorporated. In the bowl of a stand mixer fitted with the paddle attachment, cream the butter with the sugar and salt until light and fluffy.
2. Add the egg white to the bowl and mix until smooth, then add the vanilla and continue mixing. Add the flour mixture and mix until a dough holds together. Transfer to a clean work surface and knead until smooth. Divide the dough in half and form each portion into a disk.
3. Working with 1 disk at a time, roll each one between 2 sheets of parchment paper to a thickness of ⅛ inch. Transfer both to the freezer and freeze for 1 hour.
4. Working with 1 sheet at a time, peel back the parchment and using a 2-inch round or fluted cutter, cut out 24 cookies from each sheet and place on the prepared baking sheets.
5. Bake the cookies, rotating the trays top to bottom and front to back, until firm to the touch, 12 to 14 minutes. Let cool completely.
6. Meanwhile, make the filling: In the bowl of the stand mixer fitted with the paddle attachment, beat all of the filling ingredients together until light and fluffy, 3 to 4 minutes.
7. Spread ½ tablespoon of the filling on 1 cookie and sandwich with another. Repeat until you have 2 dozen sandwich cookies. Chill the cookies for 1 hour until the filling is firm, 30 minutes, then serve.