In a very large bowl stir together the cereal, nuts, and candied ginger. In a small saucepan heat chocolate-hazelnut spread, butter, and ground ginger over low heat until melted. Remove from heat and stir in milk until smooth. Pour over cereal mixture and stir gently to coat. Sprinkle with powdered sugar and cocoa powder; stir until coated. Cover and store at room temperature up to 1 week.