Pour 2 cups low-sodium chicken broth (or vegetable broth) into the Instant Pot inner pot; add 2 teaspoons minced garlic, 1/2 teaspoon kosher salt, and 1/2 teaspoon black pepper and stir to combine.
Layer 8 ounces of fettuccine noodles broken in half into the broth, arranging them in a crisscross pattern so they lie evenly and are less likely to stick.
Scatter 1 tablespoon unsalted butter (cut into small chunks) evenly over the noodles, then pour 1 cup heavy cream over the top.
Secure the lid and set the valve to Sealing. Cook on high pressure (Manual/Pressure Cook) for 6 minutes.
When the cook time ends, let the pressure release naturally for 5 minutes, then carefully move the valve to Venting to release remaining pressure slowly.
Open the lid, stir the pasta into the sauce. It may be slightly saucy; the noodles will absorb some liquid as they rest.
Stir in 1/2 cup freshly grated Parmesan cheese, then replace the lid and let the pot sit for a couple of minutes to melt the cheese into the sauce before serving.
Serve immediately, adding extra Parmesan if desired.