Step 1Combine the sliced strawberries and 2 tablespoons of the sugar. Scrape the seeds from one-half vanilla bean over the strawberries. Stir together, cover and let stand while preparing the waffles.
Step 2Toast the hazelnuts in a 350-degree oven until golden brown, about 15 to 20 minutes. Let cool to room temperature in the baking pan. Coarsely chop one-half cup plus 2 tablespoons of the toasted nuts and set aside. Process the remaining 1 cup of the nuts in a food processor until finely ground.
Step 3Stir together the ground hazelnuts, flour, remaining 2 tablespoons sugar, baking powder, salt and baking soda in a bowl.
Step 4Lightly beat together the buttermilk and egg yolks. Split the remaining vanilla bean in half and scrape the seeds into the buttermilk mixture.
Step 5Make a well in the center of the dry ingredients and pour in the buttermilk mixture. Stir just until all ingredients are almost combined, then stir in the melted butter.
Step 6Beat the egg whites to soft peaks and fold into the batter.
Step 7Heat a nonstick Belgian waffle iron according to manufacturer's directions, then spray with nonstick cooking spray or brush with oil. Pour the recommended amount of batter onto the iron. Sprinkle about 1 tablespoon chopped hazelnuts over the batter. Close the lid and cook until the waffles are browned and release easily from the iron. Repeat with the remaining batter.
Step 8To serve, spoon the sliced sweetened strawberries over the waffles and sprinkle with the remaining chopped hazelnuts.