Bring plenty of salted water to the boil in a large saucepan.
Meanwhile, in a frying pan over medium-low heat, melt the butter. Add the hazelnuts, stir and cook for 3 to 5 minutes. Stir from time to time and cook until the butter is golden and frothy. Do not burn! Remove from heat, season with salt and pepper and set aside.
Cook the ravioli in boiling water for 2 to 5 minutes, or just until al dente.
Drain and transfer in the pan, coat in the sauce and add balsamic vinegar. Cook a few seconds until well coated, then serve with a sprinkle of grated Parmesan cheese.