How to Make It
In the bowl of a stand mixer fitted with the paddle, mix the flour, cocoa, sugar, salt and baking soda. Add the butter and mix at low speed until the dough is sandy, about 10 minutes. Transfer the dough to a sheet of parchment paper, cover with another sheet of parchment paper and roll out 1/4 inch thick. Transfer to a cookie sheet and refrigerate until completely chilled, about 1 hour, or freeze.
Preheat the oven to 350°. Stamp out cookies using a 1 1/2-inch round cutter and place them on 2 parchment-lined baking sheets 2 inches apart. Bake 1 sheet at a time for about 12 minutes, until slightly firm. Transfer to a rack to cool completely.
In a bowl, cream the butter with the salt. Add the confectioners’ sugar, vanilla and milk and beat until light and fluffy. Transfer the filling to a piping bag fitted with a round medium tip.
Pipe a mound of filling on the underside of half of the cookies. Top with the remaining cookies and press lightly. Refrigerate until the filling has firmed up, about 1 hour.