chocolate-hazelnut spread, such as Nutella®
1Preheat oven to 350°F. Mix flour, cocoa powder, baking powder and salt in medium bowl. Set aside. Beat butter and granulated sugar in large bowl with electric mixer on medium speed until light and fluffy. Add egg and vanilla; mix well. Gradually beat in flour mixture on low speed until well mixed. Divide dough in half. Form each half into a log about 9 inches long and 1 1/2 inches in diameter. Wrap in plastic wrap.
2Refrigerate 1 hour or until firm. Cut dough into 1/4-inch thick slices with sharp knife. Place on ungreased baking sheets.
3Bake 10 minutes or until cookies are set. Cool on baking sheets 1 minute. Remove to wire racks; cool completely.
4To Assemble Sandwich Cookies, place about 1 tablespoon chocolate-hazelnut spread on the flat side of 1 cookie. Top with a second cookie, pressing gently to spread the filling. Repeat with remaining cookies. If desired, sandwich cookies may be refrigerated for 30 minutes to allow chocolate-hazelnut spread to become firm. Sprinkle cookies with confectioners’ sugar.