- In a very large bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add the 1-1/4 cups granulated sugar, the baking powder, and salt. Beat until well mixed, scraping side of bowl occasionally. Beat in melted chocolate until combined. Beat in eggs, milk, and vanilla. Beat in cocoa powder. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour. Divide dough in half. Cover; chill about 1 hour or until dough is easy to handle.
- Preheat oven to 375 degrees F. On a lightly floured surface, roll one portion of the dough into a 13-inch square. If desired, use a fluted pastry wheel to trim the edges of the dough square. Using fluted pastry wheel, cut dough square into twenty-five 2-1/2- to 3-inch squares. Using a 1-1/2-inch tree, bell, or snowman cutter, make a cutout in the center of half of the squares.* Place dough squares on ungreased cookie sheet.
- If desired, in a small bowl, whisk together egg white and the water; brush tops of squares lightly with egg white mixture and sprinkle with coarse sugar. Bake for 8 to 10 minutes or just until tops are firm. Let stand on cookie sheet for 1 minute. Transfer to a wire rack; let cool.
- To assemble smores, break each chocolate bar in half crosswise; place one half chocolate bar atop each solid cookie. Top each with a marshmallow. If using roasted marshmallows, top marshmallows with a cutout cookie; press down gently and serve immediately. If using unroasted marshmallows, arrange about six** of the marshmallow-topped cookies on a microwave-safe plate. Microwave on 100% power (high) for 20 to 30 seconds or until marshmallows are puffed and chocolate just starts to melt on the edges. Top each with a cookie with a cutout center. Press down gently and serve immediately. Repeat with remaining cookies, chocolate, and marshmallows. Makes 25 s'mores.
From the Test Kitchen
Bake small dough cutouts on an ungreased cookie sheet in the 375 degrees F oven for 3 to 5 minutes or just until tops are firm. Transfer to wire racks; let cool.
To microwave one cookie at a time, cook on 100% power (high) about 10 seconds.
Prepare cookies as directed through Step 3. Layer baked cookies between pieces of waxed paper in an airtight container; cover. Store at room temperature for up to 3 days or freeze for up to 3 months. Thaw cookies, if frozen. Assemble s'mores as directed in Step 4.