Chocolate Peppermint Sandwich Cookies

Adjust oven racks to upper-middle and lower-middle positions and preheat oven to 325°F (190°C). In a medium bowl, whisk flour, cocoa powder, salt, and baking powder until well combined. Set aside.

In a large bowl with a hand mixer, or in the bowl of a stand mixer fitted with a paddle attachment, beat coconut oil, granulated sugar, and butter on medium speed until fluffy, about 2 minutes, stopping to scrape down sides of bowl as needed. Add egg yolk, cream, vanilla, and peppermint extract on medium speed until combined, about 15 seconds.

With mixer running on low speed, gradually add reserved flour mixture, beating just until a dough forms, 1 to 2 minutes, stopping to scrape down sides of bowl as needed.

Transfer dough to a clean work surface and place between 2 pieces of parchment paper. With a rolling pin, roll dough to 1/8-inch thickness. Transfer dough to a baking sheet and freeze until firm, 5 to 10 minutes.

Remove dough from baking sheet and transfer to kitchen counter. Remove top sheet of parchment. Line 2 rimmed baking sheets with parchment paper and set aside. Using a 2 1/4-inch fluted or plain round cutter, cut out enough cookies to fit on both prepared sheets (about 14 to 18 cookies per sheet), spacing them about 3/4 inch apart. If the dough softens during cutting, return it to the freezer and chill until firm.

Using a 1-inch fluted cutter (or a small holiday-themed cutter), cut out center from half of the cookies. A toothpick can help lift out the small cutouts, if needed.

Bake 2 sheets at a time until cookies are set, about 12 minutes, rotating pans back to front and top to bottom halfway through baking. Transfer cookies to wire racks and allow to cool completely, 30 to 60 minutes.

While first batch of cookies bake, gather scraps, gently pressing them together, and reroll between two sheets of parchment. Freeze until firm, 5 to 10 minutes. Repeat cutting and baking remaining cookie dough until all of cookie dough is used, for a total of 44 to 52 cookies, making sure baking sheets have fully cooled before transferring cookies to them.

In a large bowl with a hand mixer, or in the bowl of a stand mixer fitted with a paddle attachment, beat butter, vanilla, salt, and peppermint extract on medium speed until smooth and creamy, 1 to 2 minutes. With mixer running on low speed, gradually add powdered sugar and 2 tablespoons heavy cream, beating until just combined, 30 to 60 seconds. Beat on medium-high speed until fluffy, about 2 minutes, adding remaining 1 tablespoon cream, 1 teaspoon at a time, until frosting reaches a pipeable consistency. Using a flexible rubber spatula, fold in peppermint candies until evenly distributed.

Transfer frosting to a piping bag fitted with a 1/2-inch round tip. Pipe filling over the flat sides of the whole (uncut) cookies, smoothing the tops with an offset spatula. Place the cut-out cookies on top, flat sides down, to form sandwiches. Garnish with additional crushed peppermint.