1/4 pound unsalted butter, cut into small pieces, plus more for cookie sheet
1/2 cup superfine sugar
1 cup unbleached all-purpose flour
1/4 teaspoon salt
3 large egg whites
1/2 teaspoon pure vanilla extract
Zest of 1 lemon, grated
8 ounces bittersweet chocolate, chopped
1 tablespoon corn syrup
1 cup chopped pistachios
Heat oven to 400 degrees. Lightly butter a heavy cookie sheet. In bowl of an electric mixer fitted with the paddle attachment, cream the butter. Add the sugar, and continue to beat until creamy, about 2 to 3 minutes. Sift the flour three times with the salt. Beat egg whites into the butter, one at a time. Add the vanilla. Scrape down the sides of the bowl with a rubber spatula. Add flour mixture gradually, scraping down the sides. Stir in the lemon zest.
Transfer the batter to a large pastry bag fitted with a #2B tip and a large-size coupler. Pipe out cookies in an S shape, about 3 inches long. Repeat S pattern in reverse.
Bake until the edges are golden brown, about 12 to 15 minutes. Transfer to a cooling rack. Cookies will become crisp as they cool.
Combine the chocolate and corn syrup in a double boiler over simmering water; stir occasionally with a rubber spatula until smooth. Using a small offset spatula or knife, spread cookie with chocolate, and top with another cookie to form a sandwich. Dip about 1 inch of each cooled cookie into chocolate, and roll to coat with pistachios; let dry.