Step 1In a large bowl, sift together the flour and sugar. Stir in the chopped nuts until evenly combined, then blend in one-half cup butter using a pastry blender. Cover and refrigerate the dough until well-chilled, at least 1 hour.
Step 2Heat the oven to 375 degrees. On a floured surface, roll the dough to a thickness of slightly less than one-eighth inch, and cut into 2-inch rounds.
Step 3Place the cookies on an ungreased baking sheet, approximately 1 inch apart, and bake until barely browned, about 7 to 10 minutes.
Step 4Remove the cookies to a rack to cool completely.
Step 5While the cookies are cooling, make the filling: In a small bowl, mix together the 2 remaining tablespoons butter with the powdered sugar and melted chocolate until fully combined.
Step 6Spread one-half teaspoon filling over the base of 1 cookie. Place a second cookie over the filling to make a sandwich. Repeat until all the cookie sandwiches are made. Store the cookies in an airtight container. The cookies can be made up to 2 months ahead (unfrosted) and frozen; thaw before frosting.