1. Process 1/2 cup toasted chopped hazelnuts and next 4 ingredients in a food processor until nuts are finely ground. Add butter, vanilla, and egg yolk; pulse until butter is cut in evenly and mixture starts to come together, scraping down sides of machine once or twice. (Dough will be very crumbly and may not come together fully until kneaded.)
2. Remove dough from processor, and knead lightly 2 or 3 times. Flatten into a disc, wrap with plastic wrap, and chill at least 2 hours or up to 3 days. Remove from refrigerator 10 minutes prior to rolling.
3. Preheat oven to 350°. Roll dough to 1/4-inch thickness on a lightly floured surface. Cut heart shapes with a 2 1/2-inch cookie cutter, and place on a parchment paper-lined baking sheet. Gather and reroll dough scraps, and cut again.
4. Bake 10 to 12 minutes or just until edges turn light golden brown. Remove from oven, transfer cookies to a wire rack, and cool completely.
5. Melt chocolate in a double boiler over low heat, stirring often, or in a microwave at HIGH 1 1/2 to 2 minutes, stirring every 30 seconds. Cool slightly. Dip half of each cookie into melted chocolate, and place on parchment paper-lined baking sheet. Sprinkle with chopped hazelnuts, if desired, and chill about 15 minutes or just until chocolate sets. Remove from refrigerator, and store in a single layer in an airtight container up to 2 days.
*To peel hazelnuts, roast at 350° for 6 to 8 minutes, and rub in a towel while warm to remove skins.
**For testing purposes, we used Scharffen Berger chocolate.
You can make the dough ahead, wrap it well with plastic wrap, and freeze up to four months. Thaw in refrigerator overnight before cutting and baking.