Makes 16 to 18 sandwich cookies
1 cup (2 sticks) unsalted butter, melted and cooled slightly
3/4 cup granulated sugar
1 teaspoon vanilla extract
1 cup semisweet chocolate chips, melted and cooled slightly
1 1/2 cups flour
3/4 cup Dutch-processed cocoa powder
1 teaspoon kosher salt
1/2 teaspoon baking soda
1. In a bowl, whisk together the butter and granulated sugar until well combined. Whisk in the vanilla and chocolate. Add the egg and whisk until thoroughly incorporated.
2. In another bowl, stir together the flour, cocoa powder, salt, and baking soda until well blended. Using a wooden spoon, stir the flour mixture into the chocolate mixture. The dough will start to seem too floury, and you will find it easiest to switch to mixing it with your hands until it comes together. It will have the consistency of Play-Doh. Let the dough sit at room temperature for about 1 hour to firm up.
3. Tear off a 15-inch-square sheet of parchment or waxed paper. Set the dough on it. Using your hands, shape the dough into a rough log about 10 inches long and 2 1/2 inches in diameter. Place the log at the edge of the paper, and roll the paper around the log. With the log fully encased in paper, roll it into a smoother log, keeping it 2 1/2 inches in diameter.
4. Refrigerate for at least 2 hours, or until firm. The log may settle and sink a bit in the fridge, so reroll it every 15 minutes or so to maintain a round log.
5. Position a rack in the center of the oven. Set the oven at 325 degrees. Line a baking sheet with parchment paper.
6. Cut the log into 1/4-inch-thick slices. Place the slices about 1 inch apart on the baking sheet. Bake for 20 to 25 minutes, or until the cookies are firm to the touch. Check them often after 16 or 17 minutes, poking them in the middle. As soon as they feel firm to the touch, remove them from the oven. You can’t judge by the color because they start out black.
7. Cool on the baking sheet.
1/2 cup (1 stick) unsalted butter, at room temperature
1 2/3 cups confectioners’ sugar
1 teaspoon vanilla extract
1 tablespoon milk
Pinch of kosher salt
1. In an electric mixer fitted with the paddle attachment or whisk, beat the butter on low speed for 30 seconds, or until the butter is completely smooth and soft. Add the confectioners’ sugar and vanilla and beat until the mixture is perfectly smooth. Add the milk and salt and beat again. The filling will look like white spackle and feel like putty. You should have about 1 cup.
2. Scoop about 1 rounded tablespoon of filling onto the bottom of 1 cookie. Top with a second cookie, bottom-side down, then press the cookies together and spread the filling toward the edges. Repeat until all of the cookies are filled. Adapted from “Flour: Spectacular Recipes From Boston’s Flour Bakery and Cafe’’