Preheat oven to 350 degrees F (175 degrees C). Grease 2 baking sheets.
Using an electric mixer, beat 6 tablespoons butter and 1/4 cup sugar in a large bowl until creamy. Add the egg; beat until smooth. Beat in corn syrup. Whisk together the flour and cocoa in a small bowl. Beat half of the flour mixture into the butter mixture until well incorporated. Beat in the remaining flour. Divide dough in half.
On a lightly floured board, roll each half of the dough into a rectangle. Cut each large rectangle into 1x3 inch rectangles. Place cookies on prepared baking sheets, 2 inches apart. Bake until firm, 10 to 15 minutes. Let cookies stand on baking sheets for 5 minutes; remove to wire racks to cool completely.
While the cookies are baking, make the filling. Heat chocolate chips in a small saucepan on lowest heat. Stir until chocolate is almost completely melted. Remove from heat; stir until smooth. Allow to cool to room temperature. Beat 1 cup confectioners' sugar and 1/2 cup butter in a large bowl until smooth. Scrape the melted chocolate into the bowl; beat well.
Spread 1/2 teaspoon filling on the bottom of 30 cooled cookies; top with remaining cookies. Sprinkle filled cookies with confectioners' sugar.