Preheat the oven to 350° F. Put the hazelnuts on a baking sheet and toast them in the oven for 15 minutes until browned and fragrant. Let cool. Prepare a large baking sheet by lining it with parchment paper. Melt the butter in a heavy-bottomed saucepan set over medium heat. Whisk in the sugar and corn syrup, and bring to a boil. Continue whisking constantly until the syrup is a rich amber color, about 10 minutes. Remove the pan from the heat and carefully whisk in the baking soda and salt. Stir in the hazelnuts and then pour the contents of the pan onto the prepared baking sheet and spread it out into a thin layer. Let the brittle cool completely. Break the brittle into 2-inch pieces. The brittle will keep in an airtight container for up to 1 week.