Directions

1. In the bowl of an electric mixer fitted with the paddle attachment, cream butter and sugar on medium speed until completely smooth, about 3 minutes. Add egg, and continue mixing until well combined.
2. In a medium bowl, whisk together cocoa powder and flour.
3. Add flour mixture to butter mixture; mix on low speed until combined, scraping down sides of bowl once. Divide dough in half; wrap each half in plastic. Flatten each half into a disk; chill until firm, at least 1 hour.
4. Preheat oven to 350 degrees. Line two baking sheets with Silpat baking mats or parchment paper. Set prepared pans aside.
5. Lightly sprinkle a clean work surface with confectioners’ sugar. Roll out dough to an 1/8-inch thickness. Using a 2-inch cookie cutter, cut out cookies, and transfer to prepared baking sheets, spacing cookies 1/2 inch apart. Chill any trimmings, and roll out again.
6. Bake the cookies until you can smell chocolate, about 12 minutes. Transfer the baking sheets to a wire rack to cool.
7. Bring cream to a boil in a small saucepan set over medium-high heat. Add chocolate to hot cream. Stir with a rubber spatula until chocolate has melted and mixture is smooth. Add peppermint oil. Remove from heat; let ganache cool slightly, about 20 minutes.
8. Transfer chocolate to a resealable plastic bag; seal, and cut off one corner at an angle. Pipe about 1 teaspoon onto the center of half of the wafers. Top with the remaining wafers. Chill the assembled cookies until firm, about 10 minutes. Store in an airtight container up to 2 days.