Preheat oven to 325°. Place hazelnuts and 1/4 cup sugar in a food processor; process until nuts are finely ground. Add almond paste, egg whites and remaining sugar; process until blended, about 3 minutes.
Cut a small hole in the tip of a pastry bag; insert a large star pastry tip. Fill bag with hazelnut mixture. Pipe 1-1/4-in. rosettes 2 in. apart onto parchment-lined baking sheets.
Bake until light brown, 12-14 minutes. Cool on pans 1 minute. Remove to wire racks to cool completely.
Dip bottom of each cookie in melted chocolate; allow excess to drip off. Place on waxed paper-lined baking sheets; refrigerate until set, about 1 hour.