1. Cook the spaghetti in a large pot of salted boiling water for 8-10 minutes, or according to packet instructions.
2. Drain the cooked pasta, reserving ½ cup of the cooking liquid. Set both aside.
3. Place the pot back over a low heat and add the butter, Vegemite and ¼ cup of the reserved cooking liquid; whisking well until combined.
4. Return the spaghetti to the pot and toss through the sauce. Add the Parmesan and remaining cooking water if needed and toss until well combined.
5. Divide between serving bowls and season with freshly ground pepper, extra grated parmesan and chopped chives.