Recipe by Jessie Sheehan
Start to Finish: 1 hour (plus 2 hours for chilling)
Servings: Makes about 20 sandwich cookies
1 1/4 cups flour
3/4 cup Dutch-process cocoa powder
1/2 tsp. salt
1/4 tsp. baking soda
1 1/4 cups sugar
1 stick unsalted butter, softened
1/4 cup vegetable shortening
2 tsp. pure vanilla extract
2 tbsp. light corn syrup
1 1/2 cups vegetable shortening
1 1/2 tsp. peppermint extract, or more to taste
1 tsp. salt
6 cups confectioners’ sugar, sifted if lumpy
A few drops of green food coloring
Mint green sparkling sugar, for decorating
1. For the cookies, in a medium bowl, whisk the flour, cocoa powder, salt, and baking soda. In a large bowl, using an electric mixer or a stand mixer fitted with a paddle attachment, beat the sugar, butter, shortening, and vanilla on medium speed until fluffy, about 2 minutes. Scrape the sides of the bowl with a spatula. Add the corn syrup; beat until just blended. Add the dry ingredients; beat on low speed until combined.
2. Place the dough on a piece of plastic wrap. Form the dough into a 2 1/4-inch-wide log, rolling it on the counter to shape into a cylinder. Tighten the plastic wrap and twist the ends. Refrigerate the dough until firm, at least 2 hours.
3. Arrange racks in the top and bottom thirds of the oven; preheat to 350°. Line 2 baking sheets with parchment paper. Unwrap the dough. Using a sharp knife, slice the dough into 1/4-inch-thick rounds; rotate the dough as you slice to avoid flattening 1 side. Arrange the slices on the baking sheets.
4. Bake the cookies, rotating the sheets from top to bottom halfway through baking, until the tops appear dry, 11 to 12 minutes. Using a metal spatula, press down lightly on each cookie to flatten the top. Let cool.
5. For the filling, in a large bowl, using an electric mixer or a stand mixer fitted with a paddle attachment, beat the shortening, peppermint extract, and salt on medium speed until just smooth. Add the confectioners’ sugar; beat until just blended. Add 2 tbsp. room-temperature water and the food coloring. Beat the filling until just smooth and uniform in color. If you like a stiffer filling, stop now. If you prefer a fluffy filling that oozes when you take a bite, beat until fluffy, another minute or two.
6. To assemble the cookies, roll 3 tbsp. of the filling in your hands to form a ball; center on the bottom of 1 cookie. Press down on the filling to flatten until it almost reaches the edge of the cookie. Top with another cookie, bottom-side down. Press down gently until the filling reaches the edge. Repeat with the remaining cookies.
7. Place the sparkling sugar in a small bowl. Roll the edges of the cookies in the sparkling sugar.