How to Make It
In a medium bowl, sift together the flour, cocoa and salt. In a standing electric mixer fitted with a paddle, cream the butter with the sugar until fluffy. Gradually mix in the dry ingredients at low speed. Turn the dough out onto a lightly floured work surface and knead lightly. Pat the dough into an 8-inch square. Wrap in plastic and refrigerate until very firm, at least 4 hours or overnight.
Preheat the oven to 350°. Cut the dough into quarters; work with 1 piece at a time and keep the rest chilled. On a lightly floured surface, roll out the dough to an 8-inch square about 1/3 inch thick. Using a sharp knife, cut the dough in quarters. Cut each square in quarters again. Transfer the 16 square cookies to an ungreased cookie sheet and bake for about 15 minutes, or until firm. Let the cookies cool on the sheet for a few minutes, then transfer them to a rack to cool completely. Repeat with the remaining 3 pieces of dough.
In a large bowl, using a wooden spoon, beat the butter with the confectioners' sugar until thick and creamy. Spread the underside of each of 32 cookies with 2 teaspoons of the filling. Top each with a second cookie to make a sandwich.