1. In a large bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar and salt. Beat until combined, scraping side of bowl occasionally. Beat in egg and cocoa powder until combined. Beat in as much of the flour as you can with the mixer. Using a wooden spoon, stir in any remaining flour and the hazelnuts.
2. Divide dough in half. Shape each portion of dough into an 11-inch-long roll. Wrap in plastic wrap or waxed paper. Chill for 1 to 2 hours or until firm enough to slice.
3. Preheat oven to 375 degrees F. Cut rolls into 1/4-inch-thick slices. Place slices 1 inch apart on an ungreased cookie sheet. Bake in the preheated oven for 6 to 8 minutes or just until tops are firm to the touch. Transfer cookies to a wire rack; let cool.
4. In a small saucepan, combine chocolate and shortening. Cook and stir over low heat until melted and smooth. Generously drizzle melted chocolate over cookies. Let stand until chocolate sets. Makes about 60 cookies.
*Test Kitchen Tip: To toast hazelnuts (filberts), preheat oven to 350°F. Spread nuts in a single layer in a shallow baking pan. Bake in preheated oven about 10 minutes or until toasted, stirring once. Place the warm nuts on a clean kitchen towel. Rub the nuts with the towel to remove the loose skins.
To Store: Layer cookies between waxed paper in an airtight container; cover. Store at room temperature for up to 3 days or freeze for up to 3 months.