What is the best method of travel from the Malpensa Airport to the Duomo District? We arrrive about noon on a weekday.
https://www.rome2rio.com/map/Milan-Malpensa-Airport-MXP/Cadorna-Fn-M2
Train works as do other modes of transport, depending on what 'best' means to you
The Malpensa Express train is the fastest way. Make sure you board the one that is destination "Cadorna". Cost is 13 EUR, takes 37 minutes, and goes every half hour at 26 and 56 past the hour from Terminal 1.
When you get there, are you going straight to a hotel? Cadorna station might be within walking distance of your hotel.
Or are you storing bags for a few hours while sightseeing? If you need bag storage, then the Malpensa Express to Milano Centrale station makes more sense and you'd later take the Metro 4 stops to reach the actual Duomo.