Put a large pot or kettle over a medium heat. Add sugar, water, and corn syrup and bring to a boil. When mixture comes to a boil, add butter. Cook to 260 degrees F on a candy thermometer without stirring and add the sliced hazelnuts. Bring mixture to 300 degrees F and stir in salt, baking soda and vanilla. Pour mixture onto a greased baking sheet and spread out and allow to cool. Break up into pieces and decorate with melted chocolate.