Divide the chocolate-hazelnut spread evenly among 4 slices of the bread and spread to cover. Sprinkle with the chopped hazelnuts. Top with the remaining slices of bread to make 4 sandwiches.
Heat a medium skillet over medium-low heat and melt about half the butter. Working in batches, cook the sandwiches, pressing down lightly as they cook until the bread toasts, about 3 minutes. Turn and toast the other side, adding butter as needed. Dust the sandwiches with confectioners' sugar and serve.
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