These decorative cookies make beautiful and tasty gifts. An important tip: be sure to chop the nuts and chocolate very fine in a food processor, or the dough will be too rough-textured to roll out smoothly. You need a graduated set of heart-shaped cookie cutters to make these.
Sift the flour, cornstarch, and salt into a medium bowl. In a large bowl, beat the butter and confectioners' sugar with an electric mixer set on high speed until combined, about 1 minute. Beat in the egg and vanilla, scraping down the sides of the bowl as needed. With the mixer on low speed, mix in the flour mixture, just until combined. Mix in the hazelnuts and chocolate.
Gather up the dough and shape into a flat disk. Wrap in plastic wrap and refrigerate until chilled, at least 2 hours. Line 2 large rimmed baking sheets with parchment paper. On a lightly floured work surface, roll out the dough to a round about ¼-inch thick. Use a 3 ½ -nch heart-shaped cookie cutter to cut out the cookies as close as possible to avoid excess scraps. Arrange the hearts about 1 inch apart on the prepared baking sheets. Gather up the dough scraps and gently knead together, cut out more cookies. If the dough becomes too soft to roll out, refrigerate until chilled. You should have about 40 cookies. Refrigerate for 30 minutes to 1 hour.
Position the ovean racks in the top third and center of the oven and preheat the oven to 325 degrees.
Use a 1-inch heart-shapped cookie cutter to cut out the centers of 20 cookies. These will be the cookie tops. Bake, switching the position of the baking sheets from top to bottom and front to back halfway through baking, until the cookies begin to brown, 20-25 minutes. Sift soft of the ½ cup of confectioners' sugar over the hot cut-out cookies. Let cool completely on the cookie sheets. Spread the jam on the cookie bottoms, leaving an 1/8-inch border around the edges. Add the tops, sugared sides up, and press together gently. Just before serving, sift the remaining confectioners' sugar over the cookies.