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1
To make the cookies: In a medium bowl, sift the flour, cocoa powder, baking soda, baking powder and salt together (I rarely sift, but I think it's worth it when it comes to cocoa powder, which can be quite lumpy). In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar on medium speed until light and fluffy, about 3-4 minutes. Add the egg and beat until combined. Mix in the buttermilk and peppermint extract. With the mixer on low, gradually add the dry ingredients, beating just until combined. Transfer the mixer bowl to your refrigerator and chill the dough for 30 minutes.
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2
Meanwhile, preheat oven to 350 F. Line two baking sheets with parchment paper.
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3
Using a small cookie scoop, portion the batter onto one of the prepared baking sheets, spacing the cookies about 2 inches apart. Bake (one sheet at a time) for about 10 minutes, or until the cookies spring back when gently pressed. Transfer the baking sheet to a wire rack and let the cookies cool for a few minutes then remove the cookies to the wire rack to cool completely. Repeat with all of the dough, alternating baking sheets.
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4
To make the filling: In the bowl of a stand mixer fitted with the paddle attachment, beat the confectioners' sugar and butter (first on low and then increasing the speed to medium) until evenly distributed and crumbly, about 1-2 minutes. Add the cream, extracts, and salt and beat on high speed until light, fluffy and smooth, about 3 minutes. If you want green buttercream, beat in a little bit of food coloring at a time until you achieve your desired color.
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5
Match the cookies in pairs by size. Transfer the filling to a piping bag and pipe a small amount onto the flat side of one cookie of each pair. Sandwich the cookies together, and press the filling to the edges.
