Preheat oven to 350°. Grease a 13x9-in. baking pan., In a large bowl, combine cake mix, eggs, water, Nutella and oil; beat on low speed 30 seconds. Beat on medium 2 minutes. Fold in chocolate chips, hazelnuts and, if desired, toffee bits. Transfer to prepared pan. Bake until a toothpick inserted in center comes out clean, 25-30 minutes., Cool completely in pan on a wire rack. Dust with confectioners' sugar if desired.