Add the uncooked pasta, water, butter, garlic powder, and a pinch of salt to the Instant Pot. Then, place the broccoli florets on top of the pasta.
Secure the lid on the pot and set the valve to sealing. Cook on manual function, high pressure, for 5 minutes or for half of the time it says on the box. (If it suggests a cook time of 8 minutes, cook the pasta for 4. If the number is off, round down, and cut the time in half.)
When the time is up, carefully do a quick release. Open the lid and add the milk and Dijon. Finally, add the cheese to the pot in 3-4 handfuls, stirring in between each addition until smooth and creamy. If you want a runnier mac, simply stir in more milk. Season with salt and pepper to taste.