Combine raspberries, sugar, and water in a medium saucepan over medium heat. Cook until the berries start to break down and release their juices, stirring occasionally. You may want to crush the berries with your spoon as you stir, to make things more uniform. When desired consistency is reached, removed from heat and set aside.
For the waffles
hazelnuts, plus more for serving
hazelnut liqueur (Frangelico)
Preheat oven to 325° F. Spread hazelnuts on a baking sheet and toast for about 10 minutes in the oven, until fragrant. Remove and let cool.
Preheat waffle iron. Pulse hazelnuts in a food processor until you achieve a powder-like consistency. Be careful not to overprocess or you will end up with hazelnut butter!
In a large bowl, whisk together egg yolks, buttermilk, vegetable oil, and hazelnut liqueur. In a medium bowl, whisk together flour, cornstarch, baking powder, and salt. Set both bowls aside.
In the bowl of a stand mixer fitted with the whisk attachment, beat egg whites and sugar until stiff peaks form.
Add dry ingredients to buttermilk mixture and stir until just combined. Gently fold in egg whites until batter is uniform. Pour batter into waffle iron and cook according to the manufacturer's instructions.
Serve with raspberry syrup from above (you can heat it up over the stove again if it is a little thick) and chopped hazelnuts.