Preheat oven to 350°F. Line 2 cookie sheets with parchment paper; set aside. In a large mixing bowl beat butter with an electric mixer on medium to high speed for 30 seconds. Add brown sugar and beat until combined. Add egg yolk, vanilla, and salt; beat until combined. Add the flour and beat on low speed until dough comes together (may appear crumbly but will come together). Stir in chopped chocolate.
Shape dough into 24 1 1/4-inch balls. Arrange 2 inches apart on prepared cookie sheets. Use your thumb to make indents in each ball. Fill each with 1/2 teaspoon chocolate-hazelnut spread.
Bake about 10 minutes or until edges are lightly browned. Remove and cool on wire racks.