Secure lid of Instant Pot, making sure valve is turned to seal. Set on Pressure Cook (or Manual high pressure, depending on your model) for 4 minutes. It will take about 15 minutes to come to pressure.
When cooking has finished, quick release pressure. The valve tends to sputter when you release the pressure. I place a towel over the valve to prevent spatters; be careful when removing towel because it may be hot. When the pressure is completely released (valve is lowered), carefully remove cover.
Keeping the Instant Pot on the Warm setting, stir in evaporated milk, mustard powder, and optional garlic powder. One cup at a time, stir in shredded cheddar and Parmesan cheese. Continue stirring until cheese is melted.
1 can (12 ounces) evaporated milk, ½ teaspoon ground dry mustard, ¼ teaspoon garlic powder, optional, 3 cups shredded sharp cheddar cheese, ½ cup shredded Parmesan cheese
Taste and season with ground black pepper, as desired. Serve immediately.
Video
Notes
Shredded cheese: I prefer to shred my own cheddar cheese. I find it yields a smoother, velvety result. I have used pre-shredded, though, and it also works. It doesn’t stay quite as smooth as it cools, though.
Baking instructions: Place cooked mac & cheese in greased baking dish. If desired, top with additional cheese, cracker crumbs, or buttered bread crumbs. Bake at 350ºF for 20 to 30 minutes or until golden brown and a little crispy on the top.
Storage & reheating: Refrigerate leftover mac and cheese in a covered container for up to 5 days. To reheat, microwave individual portions until just warmed through, or heat gently in a saucepan over medium low heat. Stir in additional milk if the mac and cheese seems too dry.