You can use Dutched or natural cocoa powder and nonfat or whole dry milk powder here. To ensure that your cookies are the same size, it's important to roll the logs so that they have a diameter of 1½ inches. Don't skip the rest in step 7; letting the sliced dough warm to room temperature helps the cookies expand evenly as they bake.

Whisk flour, salt, and baking powder together in bowl. In second bowl, whisk melted butter, granulated sugar, egg yolk, and peppermint extract until very smooth. Add flour mixture and stir with rubber spatula or wooden spoon until well combined (dough will be loose). Let dough rest until firm, about 5 minutes.

Shape dough into rough 8 by 5-inch rectangle. Wrap tightly in plastic wrap and refrigerate until firm, at least 1 hour or up to 3 days.

Sift espresso powder, cocoa, confectioners' sugar, and milk powder through fine-mesh strainer into medium bowl. Cut chilled dough into rough ¾-inch pieces. Add to espresso-cocoa mixture and toss until pieces are evenly coated. Transfer pieces to counter, shaking excess espresso-cocoa mixture back into bowl (do not discard). Divide dough pieces into 2 piles.

Working with 1 pile, firmly squeeze one-quarter of pieces together to form dough ball with streaks of espresso mixture throughout (do not knead dough). Repeat with remaining three-quarters of dough to form 3 more balls. Squeeze and press dough balls together to form rough 8-inch log. Continue to shape and roll log until 1½ inches in diameter and about 9 inches long. Flatten log slightly on 2 sides, until it is 1 inch tall and has oval shape. Repeat with remaining dough pile to form second log.

Transfer reserved espresso-cocoa mixture to counter and roll logs in mixture to coat evenly. Wrap logs tightly in plastic and refrigerate until firm, about 30 minutes.

Adjust oven rack to middle position and heat oven to 350 degrees. Line 2 rimmed baking sheets with parchment paper.

Slice logs into ¼-inch-thick ovals and space 1 inch apart on prepared sheets. Let rest at room temperature for 10 minutes. Bake cookies, 1 sheet at a time, until puffed and tops look dry but centers are still very soft, 10 to 12 minutes, rotating sheet halfway through baking.

Let cookies cool on sheet for 10 minutes, then transfer to wire rack and let cool completely, about 30 minutes. Serve. (Cookies can be stored at room temperature for up to 10 days.)

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