Set the spaghetti noodles in the pot. Add the broth and 2 tablespoons of butter. Make sure the pasta is completely submerged.
Lock on the lid and set the vent to sealing. Pressure cook on high for 4 minutes. When done, do a quick release (be careful when venting as this is a starchy pasta).
Open the lid and stir the pasta. It will still be quite liquidy. Let sit on Keep Warm for 3-5 minutes, stirring occasionally, until liquid is halved. Add in the salt and remaining butter. Stir to combine. Allow the pasta to sit in the Instant Pot on Keep Warm for a few minutes until your desired consistency is reached. It will thicken as it sits. Stirring helps soak up the liquid.
Garnish with Parmesan cheese, if desired.
Notes
Add any type of cheese, if you want.
Use any type of pasta you like. No additional cook time is necessary.
You can double the recipe just by doubling the ingredients. No additional cook time is necessary.