Preheat oven to 350°F. Prepare cookie sheet with parchment paper.

In medium bowl, combine flour, sugar, baking powder and salt.

In large bowl, beat butter, eggs, and vanilla with electric mixer at medium speed until well combined. With electric mixer on low speed, add flour mixture to egg mixture 1 cup at a time, scraping bottom and sides of bowl as needed. Mix until just combined. Beat in hazelnuts. On lightly floured surface, pat into a 10 in. x 2 in. X 1 in. log; place in middle of prepared cookie sheet.

Bake 25 to 28 minutes or until firm to the touch. Cool log 15 minutes on cookie sheet. Slice into 1/2 in. thick pieces with serrated knife. Place slices cut side up on cookie sheet. Reduce oven temperature to 300°F. Bake cut biscotti 10 minutes; flip biscotti and bake an additional 10 minutes or until firm. Cool completely on cookie sheet.

Melt Candy Melts candy according to package instructions. Divide candy into five parts and color four of them with candy color to make violet, green, pink and yellow. Add some of each color to the white candy and swirl to make a marble design. Dip bottoms of biscotti in melted candy. Place on parchment-lined cookie sheet until set, 5-10 minutes. Continue with remaining biscotti, adding more color candy drops as necessary to renew the marble design.