01 - Preheat oven to 350°F. Line two baking sheets with parchment paper and set aside.
02 - In a medium bowl, whisk together the all-purpose flour, unsweetened cocoa powder, baking soda, and salt until evenly blended.
03 - In a large mixing bowl, beat the softened unsalted butter and granulated sugar together using an electric mixer until the mixture is pale, light, and fluffy.
04 - Add the large egg and vanilla extract to the butter mixture, beating until fully combined and smooth.
05 - Gradually add the dry ingredient mixture to the wet ingredients, mixing on low speed until just combined. Avoid overmixing.
06 - Roll tablespoonfuls of dough into balls, flatten slightly with your palm, and arrange them 2 inches apart on the prepared baking sheets.
07 - Bake for 8 to 10 minutes until the cookies are set. Allow them to rest on the baking sheet for 2 minutes, then transfer to a wire cooling rack to cool completely.
08 - In a mixing bowl, beat the softened unsalted butter until smooth. Add the sifted powdered sugar, heavy cream, peppermint extract, salt, and food coloring if using. Beat until the filling is light and fluffy.
09 - Spread or pipe approximately 1 tablespoon of peppermint filling onto the flat side of half the cooled cookies. Top each with a second cookie, flat side down, to form sandwiches.
10 - Refrigerate the assembled cookies for 15 minutes to allow the filling to firm up before serving.