Toast hazelnuts in a small dry skillet over medium-low heat, stirring, until lightly colored and fragrant, 3 to 4 minutes. Peel bananas and cut in half lengthwise. Arrange 2 banana halves on each dessert plate. Place a scoop of frozen yogurt (or ice cream) in the center of each banana split and top with Chocolate-Hazelnut Sauce. Garnish with hazelnuts and serve.