In a large bowl, beat together butter and brown sugar with a mixer at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add egg and vanilla extract, beating until combined.
In a medium bowl, whisk together flour, hazelnuts, and baking soda. With mixer at low speed, gradually add flour mixture to butter mixture, beating just until combined. (Dough will be crumbly.)
Turn out dough onto a lightly floured surface, and gently knead 5 to 7 times. Shape dough into a disk, and wrap in plastic wrap. Refrigerate for 30 minutes.
Preheat oven to 350°. Line rimmed baking sheets with parchment paper.
On a lightly floured surface, using a rolling pin, roll out dough to a ¼-inch thickness. Using a 1½-inch fluted square cutter, cut 60 cookies from dough, rerolling scraps as necessary. Using an offset spatula, transfer cookies to prepared baking sheets. Freeze for 15 minutes.
Bake until edges of cookies are lightly browned, 8 to 10 minutes. Let cool on pans for 5 minutes. Remove from pans, and let cool completely on wire racks.
In a small microwave-safe bowl, microwave chocolate chips on medium in 30-second intervals, stirring between each, until melted and smooth. Dip half of each cookie in melted chocolate, and place cookies on wax paper. Sprinkle chocolate with hazelnuts, if desired. Let chocolate set completely. Store cookies in an airtight container at room temperature until ready to serve.
Recipe by TeaTime Magazine at https://teatimemagazine.com/buttery-hazelnut-cookies/