Beat butter and sugar at medium-high speed until light and creamy, about 2 minutes. Add eggs, and
beat until smooth, about 3 minutes. Beat in vanilla.
Combine flour, baking powder, cinnamon, salt, and zest. Add to butter mixture;
beat on medium speed until combined, about 1 minute. Refrigerate for at least 30 minutes.
Form dough into 3/4-inch
balls, and roll in ground hazelnuts. Place balls on ungreased baking sheet 1 inch apart. Bake until cookies begin to set,
about 8 minutes. Remove from oven. Working quickly, make a slight indentation in each cookie with your
thumb. Return to oven, and bake